Synthesis of Petrographic, Geochemical, and Isotopic Data 

for the Boulder Batholith, Southwest Montana

U.S. Department of the Interior

U.S. Geological Survey

Professional Paper 1793
Cover photograph: Planar jointed, spheroidal weathered Butte Granite at Homestake Pass, Montana.
Synthesis of Petrographic, Geochemical, 

and Isotopic Data for the Boulder Batholith, 

Southwest Montana

By Edward A. du Bray, John N. Aleinikoff, and Karen Lund

Professional Paper 1793

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1–888–ASK–USGS.

For an overview of USGS information products, including maps, imagery, and publications, 

visit http://www.usgs.gov/pubprod

To order this and other USGS information products, visit http://store.usgs.gov

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

du Bray, E.A., Aleinikoff, J.N., and Lund, Karen, 2012, Synthesis of petrographic, geochemical, and isotopic data for 

the Boulder batholith, southwest Montana: U.S. Geological Survey Professional Paper 1793, 39 p.
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Study Description...........................................................................................................................................4

Petrographic Data..........................................................................................................................................5

Major Oxide Data...........................................................................................................................................9

Mafic Group...........................................................................................................................................9

Intermediate Group...............................................................................................................................9

Felsic Group...........................................................................................................................................9

Trace Element Data .....................................................................................................................................12

Mafic Group.........................................................................................................................................12

Intermediate Group.............................................................................................................................12

Felsic Group.........................................................................................................................................12

U-Pb Zircon Geochronology.......................................................................................................................17

Methods................................................................................................................................................17

Results ..................................................................................................................................................18

Radiogenic Isotope Data ............................................................................................................................20

Petrogenetic Implications of Geochemical and Isotopic Data ............................................................20

Conclusions...................................................................................................................................................35

Acknowledgments.......................................................................................................................................35

References Cited..........................................................................................................................................35

Figures

1. Index map showing location of Boulder batholith relative to major 

geologic features in the region ..................................................................................................2

2. Map showing the Boulder batholith, southwest Montana....................................................3

3. Quartz-alkali feldspar-plagioclase ternary diagrams showing modal 

compositions of Boulder batholith plutons...............................................................................7

4. Variation diagrams showing major oxide abundances in samples of 

the Boulder batholith, southwest Montana............................................................................11

5. Total alkali-silica variation diagram showing compositions of the 

Boulder batholith, southwest Montana ..................................................................................13

6. FeO*/(FeO*+MgO) versus SiO2 variation diagram showing compositions 

the Boulder batholith, southwest Montana............................................................................14

7. Na2O+K2O-CaO versus SiO2 variation diagram showing compositions 

of the Boulder batholith southwest Montana........................................................................15

8. Variation diagram showing molar major oxide compositions of the Boulder 

batholith, southwest Montana as a function of alumina and alkali saturation................16

9. Trace-element, tectonic-setting-discrimination variation diagram showing 

the composition of the Boulder batholith, southwest Montana .........................................18

10. Variation diagram showing relative proportions of rubidium, potassium, and 

strontium in samples of the Boulder batholith, and its peripheral plutons, 

southwest Montana ...................................................................................................................19
iv

11. Chondrite-normalized rare-earth element diagrams for Boulder batholith 

samples ........................................................................................................................................21

12. Primitive mantle-normalized trace-element diagrams for Boulder 

batholith samples........................................................................................................................22

13. Photographs of representative zircons and Tera-Wasserburg plots of 

sensitive high-resolution ion microprobe (SHRIMP) U-Pb dates from each 

sample dated in this study.........................................................................................................23

14. Graph showing epsilon neodymium versus initial strontium isotopic data 

for Boulder batholith samples ..................................................................................................33

Tables

1. Summary of petrographic characteristics for plutons of the Boulder 

batholith, southwest Montana....................................................................................................6

2. Average mineralogic and chemical compositions of Butte Granite 

north and south of latitude 46.2°N. Boulder batholith, southwest Montana ......................9

3. Average chemical compositions for plutons of the Boulder batholith, 

southwest Montana, arranged in order of increasing silica content................................10

4. SHRIMP U-Th-Pb data for zircon from plutons of Boulder batholith, 

southwest Montana ...................................................................................................................27

5. Summary of age data for plutons of Boulder batholith, southwest Montana. .................33

6. Rb-Sr and Sm-Nd isotopic data for samples of the Boulder batholith, 

southwest Montana ...................................................................................................................34

Appendix

1. Petrographic and geochemical data for samples of the Boulder 

batholith, southwest Montana............................................................................................... link
Abstract

The Late Cretaceous Boulder batholith in southwest 

Montana consists of the Butte Granite and a group of associated smaller intrusions emplaced into Mesoproterozoic to 

Mesozoic sedimentary rocks and into the Late Cretaceous Elkhorn Mountains Volcanics. The Boulder batholith is dominated 

by the voluminous Butte Granite, which is surrounded by as 

many as a dozen individually named, peripheral intrusions. 

These granodiorite, monzogranite, and minor syenogranite 

intrusions contain varying abundances of plagioclase, alkali 

feldspar, quartz, biotite, hornblende, rare clinopyroxene, and 

opaque oxide minerals. Mafic, intermediate, and felsic subsets 

of the Boulder batholith intrusions are defined principally on 

the basis of color index. Most Boulder batholith plutons have 

inequigranular to seriate textures although several are porphyritic and some are granophyric (and locally miarolitic). Most 

of these plutons are medium grained but several of the more 

felsic and granophyric intrusions are fine grained. Petrographic 

characteristics, especially relative abundances of constituent 

minerals, are distinctive and foster reasonably unambiguous 

identification of individual intrusions.

Seventeen samples from plutons of the Boulder batholith were dated by SHRIMP (Sensitive High Resolution Ion 

Microprobe) zircon U-Pb geochronology. Three samples of the 

Butte Granite show that this large pluton may be composite, 

having formed during two episodes of magmatism at about 

76.7 ± 0.5 Ma (2 samples) and 74.7 ± 0.6 million years ago 

(Ma) (1 sample). However, petrographic and chemical data 

are inconsistent with the Butte Granite consisting of separate, 

compositionally distinct intrusions. Accordingly, solidification 

of magma represented by the Butte Granite appears to have 

spanned about 2 million years (m.y.). The remaining Boulder 

batholith plutons were emplaced during a 6–10 m.y. span 

(81.7 ± 1.4 Ma to 73.7 ± 0.6 Ma).

The compositional characteristics of these plutons are 

similar to those of moderately differentiated subduction-related 

magmas. The plutons form relatively coherent, distinct but 

broadly overlapping major oxide composition clusters or linear 

arrays on geochemical variation diagrams. Rock compositions 

are subalkaline, magnesian, calc-alkalic to calcic, and metaluminous to weakly peraluminous. The Butte Granite intrusion is 

homogeneous with respect to major oxide abundances. Each of 

the plutons is also characterized by distinct trace element abundances although absolute trace element abundance variations 

are relatively minor. Limited Sr and Nd isotope data for wholerock samples of the Boulder batholith are more radiogenic 

than those for plutonic rocks of western Idaho, eastern Oregon, 

the Salmon River suture, and most of the Big Belt Mountains. 

Initial strontium (Sri

) values are low and epsilon neodymium 

(εNd ) values are comparable relative to those of other southwest 

Montana basement and Mesozoic intrusive rocks. Importantly, 

although the Boulder batholith hosts significant mineral deposits, including the world-class Butte Cu-Ag deposit, ore metal 

abundances in the Butte Granite, as well as in its peripheral 

plutons, are not elevated but are comparable to global average 

abundances in igneous rocks.

Introduction

The Boulder batholith encompasses about 4,000 square 

kilometers (km2) in southwest Montana (bounded by latitudes 

45.6° and 46.7°N. and longitudes 112.75° and 111.5°W.) and 

lies in the Helena embayment of the Mesoproterozoic Belt 

Supergroup and in the foreland of the Late Cretaceous fold and 

thrust belt (figs. 1 and 2). In this setting, the Boulder batholith 

is far to the east (inboard) of other Late Cretaceous batholiths, 

such as the Idaho batholith, of the North American Cordillera.

The batholith is the subject of many detailed studies, 

including the duration of magmatism, petrology of constituent 

plutons, and their relations to nearby volcanic rocks and associated mineral deposits, that led to modern models of batholith-scale intrusive systems. Extensive mapping and petrologic 

studies by U.S. Geological Survey scientists helped characterize the igneous rocks of the Boulder batholith region (Becraft 

and Pinkney, 1961; Becraft and others, 1963; Emmons and 

Weed, 1897; Klepper and others, 1957, 1971b, 1974; Knopf, 

1913, 1950, 1957, 1963; Robinson and others, 1968; Ruppel, 

1963; Smedes, 1966; Tilling, 1964; Tilling and others, 1968; 

and Tilling and Gottfried, 1969). These studies established 

the field relations, modal composition, and geochemistry of 

individual intrusions that constitute the batholith. Important 

related and follow-on studies pertain to (1) ages of plutonic 

rocks (Knopf, 1956, 1964; Tilling and others, 1969; Tilling, 

1974), (2) relations between the mineral deposits and their 

Synthesis of Petrographic, Geochemical, and Isotopic Data 

for the Boulder Batholith, Southwest Montana

By Edward A. du Bray, John N. Aleinikoff, and Karen Lund
2 Synthesis of Petrographic, Geochemical, and Isotopic Data for the Boulder Batholith, Southwest Montana

Figure 1. Location of Boulder batholith relative to major geologic features in the region.

Salmon River suture

Limit of fold and thrust belt

Proterozoic Belt Supergroup

Helena embayment

Idaho batholith

Idaho batholith

Pioneer

batholith

Tobacco

 Root

batholith

 Boulder

batholith

WYOMING

IDAHO

CANADA

MONTANA

Butte

UNITED STATES

Study area

116° 49°

45°

INDEX MAP

0 25 50 75 KILOMETERS

0 25 50 75 MILES

104°

Cretaceous Boulder batholith

Other Cretaceous plutons

Eocene plutons

EXPLANATION

MONTANA

N:\Jeff\den12_cmrk00_0094_pp_dubray\report_figures\figure_01.ai

Area of

figure 2
Introduction 3

0 5 10 KILOMETERS

0 5 10 MILES

E XPLANATION

Fault

Quaternary surficial deposits

Contact

Sample location

Tertiary volcanic rocks

Butte Granite

Climax Gulch granodiorite

Pulpit Rock monzogranite

Colorado Gulch granodiorite

Granophyre

Homestake monzogranite 

Felsic monzogranite

Donald granodiorite

Moose Creek monzogranite

Hell Canyon monzogranite

Deer Creek granophyre

Moosetown monzogranite

Elkhorn Mountains Volcanics

Mesozoic and Upper Paleozoic

 sedimentary rocks

Paleozoic sedimentary rocks

Precambrian rocks

Other Mesozoic intrusions

Camp Thunderbird monzogranite

Mafic intrusions

Rader Creek Granodiorite

Unionville Granodiorite

Burton Park granodiorite

N:\Jeff\den12_cmrk00_0094_pp_dubray\report_figures\figure_02.ai

112°30’ 112°15’ 112°00’ 111°45’

46°30’

46°15’

45°45’

46°00’

Butte

Base from U.S. Geological Survey digital data, 2009, 1:200,000

Universal Transverse Mercator projection

Zone 12

North American Datum of 1927 (NAD27)

Geology mapped by H.W. Smedes and others, 1988

Figure 2. Boulder batholith, southwest Montana (modified from Smedes and others, 1988).
4 Synthesis of Petrographic, Geochemical, and Isotopic Data for the Boulder Batholith, Southwest Montana

Boulder batholith host rocks (Doe and others, 1968; Knopf, 

1913; Tilling, 1977; and Weed, 1912; ), (3) the geometric and 

genetic relations between the batholith and the nearby Elkhorn 

Mountains Volcanics (Davis and others, 1963, 1965; Hamilton 

and Myers, 1967; Klepper and others, 1971a, 1974; Robinson 

and others, 1968; Rutland and others, 1989; and Tilling, 1974), 

(4) the thickness and crustal geometry of the batholith (Berger 

and others, 2011; Biehler and Bonini, 1969; and Schmidt 

and others, 1990) and (5) the isotopic and petrologic characteristics of these rocks, which have been used to define the 

petrogenetic evolution of associated magmas (Lambe, 1981; 

Tilling, 1968, 1973, and 1974).

Early studies demonstrated that the Boulder batholith 

consists of a series of related plutons (fig. 2) that were 

emplaced into Mesoproterozoic to Mesozoic sedimentary 

rocks and into reportedly cogenetic rocks of the Late Cretaceous Elkhorn Mountains Volcanics (Hamilton and Myers, 

1967; Klepper and others, 1971a, b; Lambe, 1981; Robinson 

and others, 1968; Rutland and others, 1989; and Tilling, 

1974). About 70 percent of the batholith is composed of the 

voluminous Butte Granite, but it includes as many as a dozen 

individually named, peripheral plutons (and many more named 

and unnamed exposures of plutonic rocks) that surround the 

Butte Granite.

A Cretaceous age for the Boulder batholith was established during early K-Ar dating investigations (Knopf, 1956). 

Subsequent K-Ar studies approximately defined the emplacement sequence of Boulder batholith intrusions (Tilling and 

others, 1968) and generally confirmed relative ages determined from field relations (Becraft and Pinkney, 1961; Becraft 

and others, 1963; and Ruppel, 1963). U-Pb zircon geochronologic investigations confirm that all Boulder batholith intrusions were emplaced between about 82 and 74 Ma and, with 

one exception, confirm the previously established intrusion 

sequence (Lund and others, 2002). Geochronologic data for 

the Elkhorn Mountains Volcanics are limited but a combination of fossils associated with distal and presumably cogenetic 

volcanic rocks and K-Ar ages on hornblende show that volcanism began about 80 Ma (as summarized by Tysdal, 2000) 

and may have continued until about 73 Ma (Robinson and 

others, 1968). As part of the United States northern Cordillera, 

abundant Cretaceous igneous rocks of the Boulder batholith 

and nearby region have been interpreted as byproducts of 

subduction-related processes, including back-arc magmatism, 

which prevailed along the west edge of the North American 

plate during this time interval (Christiansen and Yeats, 1992, 

fig. 1; Coney and Reynolds, 1977; and Lipman, 1992).

Mineral deposits associated with rocks of the Boulder 

batholith are the subjects of many investigations (Brimhall, 

1973, 1977, 1979, 1980; Brown, 1894; Emmons and Tower, 

1897; Weed, 1912; Knopf, 1913, 1950; Lange and Cheney, 

1971; Meyer, 1965; Meyer and others, 1968; Miller, 1973; 

Proffett, 1973; Roberts, 1975; Rusk and others, 2008; Sales, 

1913; Sales and Meyer, 1948, 1949; and Smedes, 1966 ). 

However, interpretations of relations between mineral deposits 

and host batholithic rocks have been controversial. Recently, 

Lund and others (2002) documented the temporal relations 

between magmatism and diverse types of ore deposits spatially 

associated with the Boulder batholith. That study also determined that some of the deposits are genetically related to the 

batholith but many others are related to volumetrically minor, 

younger Cretaceous intrusions or to Tertiary-age volcanic 

rocks (Lund and others, 2002; O’Neill and others, 2004; and 

Taylor and others, 2007). The mineral deposits in the Boulder 

batholith region are highly significant (see Elliot and others, 

1992; and Tysdal and others, 1996) but many questions remain 

concerning relations to various intrusive episodes, metal 

sources, and deposit ages.

Geochemical and isotopic studies have been conducted 

on selected rocks and(or) minerals of the Boulder batholith 

(Doe and Tilling, 1967; Doe and others, 1968; Greenland and 

others, 1968, 1971, 1974; Gottfried and others, 1972; Tilling, 1964, 1968, 1973, 1974, 1977; and Tilling and Gottfried, 

1969). However, with the exception of these focused studies, 

the petrology and petrogenesis of the individual plutons or of 

the batholith as a whole have not been systematically investigated. The primary goal of this report is to use geochemical 

and petrographic data compiled by du Bray and others (2009), 

in conjunction with the new SHRIMP U-Pb ages and Sr and 

Nd isotopic data, to produce a petrogenetic evaluation of 

geochemical and modal compositional variation of the Late 

Cretaceous Boulder batholith through time. Because dating 

studies indicate that significant polymetallic quartz-vein 

mineral deposits are temporally related to the Butte Granite 

(Lund and others, 2002), the geochemical data can also be 

used to investigate the metallogeny of the various Boulder 

batholith plutons.

Study Description

The data compilation of du Bray and others (2009) 

contains geochemical data for almost 400 Boulder batholith 

samples; in addition, relative abundances of major rockforming minerals in about 250 of these samples are recorded. 

Subsequent to publication of the compilation, a small number of new chemical and modal analyses were added to the 

working version (Appendix 1) of the previously published 

Boulder batholith database (du Bray and others, 2009). Using 

criteria described by du Bray and others (2009), all altered 

samples have been identified and removed from the geochemical dataset; similarly, all censored values (less than, 

greater than, and so forth) were replaced by blank cells. 

To mitigate the effects of variable postmagmatic oxidation 

processes, all iron abundances were converted to equivalent 

ferrous iron (FeO*). To eliminate disparities caused by variable volatile component contents and to facilitate meaningful 

comparison of oxide abundances, major oxide analyses were 

recalculated to 100 percent on a volatile-free basis. Although 

the compilation of du Bray and others (2009) includes analyses for numerous small stocks and plugs that intruded the 

Elkhorn Mountains Volcanics north and east of the Boulder 
Petrographic Data 5

batholith, petrogenetic relations between these rocks and 

those of the Boulder batholith are not well documented. 

Accordingly, the compositional features of the satellitic 

plutons are not further addressed.

In the present study, plutons are referred to by compositional names derived from available modal data; some 

of these names differ from previous usage. In this regard, 

the name “Butte Granite” is used as explained by Lund and 

others (2002). Location names are given for some previously unnamed plutons, including (1) monzogranite of Camp 

Thunderbird (for granodiorite, undivided, from Knopf, 1950), 

(2) granophyre of Deer Creek (for previously mapped but 

unnamed body previously called aplite), and (3) granodiorite of Colorado Gulch (for porphyritic granodiorite from 

Knopf, 1950). These and other informal names for plutons are 

retained but simplified in the text, as in Homestake monzogranite (for monzogranite of Homestake). The Butte Granite 

includes many tabular or sheet-like masses previously referred 

to as alaskite or aplite. In recognition of their distinctive chillzone characteristics, including granophyric textures, leucocratic composition, and miarolitic cavities, we refer to the 

alaskite and aplite masses as granophyre.

Petrographic Data

As summarized by Bateman (1992, page 25) a pluton 

consists of “...a body of intrusive rock that is expressed at 

the surface by a single exposure that is continuous...” and 

has “...similar composition, texture, and age...” throughout. 

According to this definition, the Boulder batholith consists of 

a series of plutons, each with distinctive petrographic characteristics (table 1). Variable color indices (sum of modal 

biotite, hornblende, pyroxene, and opaque oxide mineral 

abundances) indicate that plutons of the Boulder batholith 

form three groups: (1) mafic (Burton Park, Unionville, and 

Rader Creek), (2) intermediate (Butte, Climax Gulch, Donald, Camp Thunderbird, Hell Canyon, Homestake, Moose 

Creek, Moosetown, Colorado Gulch, and Pulpit Rock), and 

(3) felsic (Deer Creek and other granophyric masses). The 

mafic plutons have color indices (table 1) that range from 

17 to 24 and their average compositions are granodiorite [per 

the International Union of Geological Sciences classification 

scheme of Streckeisen (1976), fig. 3]. Color indices of the 

intermediate group plutons range from 6 to 13 and most plutons are composed of monzogranite (although the Donald and 

Climax Gulch plutons are composed of felsic granodiorite, 

fig. 3). The felsic plutons have color indices that range from 

1 to 2 and their average compositions are monzogranite and 

syenogranite (fig. 3). Modal data for the Boulder batholith 

plutons define coherent, mutually overlapping QAP (quartzalkali feldspar-plagioclase) composition fields. Compositions 

of single plutons vary sufficiently that some sample compositions plot in modal fields other than that of the average 

(fig. 3). Alkali-feldspar-plagioclase abundance ratios for average Boulder batholith modal compositions generally correlate 

with variations in color index among these rocks; plutons 

with the highest color indices generally have the lowest alkali 

feldspar/plagioclase.

Most Boulder batholith plutons have inequigranular to 

seriate, hypidiomorphic textures. Exceptions are the Colorado Gulch, Climax Gulch, Moosetown and Moose Creek 

plutons that contain phenocrysts, the Butte Granite (which 

almost attains a porphyritic texture in places), and sheetlike granophyre intrusions and the Deer Creek granophyre 

(which have xenomorphic, granophyric textures). The Deer 

Creek and other granophyric intrusions are fine grained but 

all other Boulder batholith rocks are medium grained. In 

Boulder batholith rocks, plagioclase is generally moderately 

oscillatory zoned and forms 2–3 millimeter (mm) euhedral to 

subhedral laths. Weakly perthitic alkali feldspar is anhedral, 

interstitial, and forms 2–mm grains. Anhedral quartz, pale 

green to olive green hornblende, and tan to dark reddishbrown biotite form anhedral grains approximately1 mm in 

greatest dimension. Although hornblende and biotite are the 

principal mafic silicate minerals in Boulder batholith plutons, trace amounts of clinopyroxene have been identified in 

the three plutons of the mafic group, and rarely in the Butte 

Granite. Plutons of the mafic group contain approximately 

equal amounts of hornblende and biotite, although hornblende 

is slightly more abundant. Intermediate-group plutons contain 

approximately equal amounts of hornblende and biotite, 

although biotite is most abundant in plutons with lower color 

indices. The Moosetown and Pulpit Rock monzogranites of 

the intermediate group are somewhat unusual in having color 

indices between 7 and 8 but contain no hornblende. Plutons 

with color indices less than 7 contain essentially no hornblende. Boulder batholith intrusions also contain 1–2 percent 

opaque oxide minerals; grains are anhedral to subhedral 

and 0.1–0.2 mm. The accessory mineral suite in these rocks 

includes combinations of zircon, apatite, titanite, and rare 

allanite. Glomerocrysts of hornblende, biotite, and opaque 

oxide minerals are common constituents of the Butte Granite 

and the granodiorites of Burton Park and Donald. The granophyric masses contain miarolitic cavities; in many places 

within the Deer Creek granophyre, these cavities contain 

chalcopyrite and molybdenite (Smedes and others, 1973).

Berger and others (2011) show that the Butte Granite is 

a composite intrusion underlain by distinct feeder conduits 

north and south of about latitude 46.2°N. A 2 m.y. age difference (see U-Pb zircon geochronology section) determined 

for Butte Granite samples from its northern and southern 

parts is also consistent with a composite intrusion. However, 

average modal compositions computed for groups of Butte 

Granite samples from north and south of latitude 46.2°N., 

respectively, indicate that the relative abundances of the 

major rock-forming minerals in these two groups of samples 

are statistically indistinguishable (table 2).
Table 1. Summary of petrographic characteristics for plutons of the Boulder batholith, southwest Montana, arranged in order of decreasing color [Plag, plagioclase; Alk fldsp, alkali feldspar, Qtz, quartz; Hb, hornblende; Pyx, pyroxene; Bi, biotite; Opq, opaque oxide minerals. TR, trace. Accessory minerals: ti, titanite; anite; mn, monazite; tr, tourmaline; xe, xenotime; gar, garnet. Blank fields constitute not present] Unit designation Pluton short name Average modal composition Plag Alk fldsp Qtz Hb Pyx Bi Opq Color index Grain size Textu

Granodiorite of Burton Park Burton Park Granodiorite 43 17 15 11 TR 11 2 24 medium Seriate, hypidiom

Unionville Granodiorite Unionville Granodiorite 40 21 19 10 TR 8 TR 18 medium Seriate, hypidiomRader Creek Granodiorite Rader Creek Granodiorite 46 18 19 9 TR 6 2 17 medium Seriate, hypidiomButte Granite Butte Monzogranite 38 26 22 5 TR 7 1 13 medium Inequigranular to 

hypidiomorph

Granodiorite of Climax Gulch Climax Gulch Granodiorite 40 20 28 4 6 2 12 medium Inequigranular, hypidiomorph

Monzogranite of Camp Thunderbird Camp Thunderbird Monzogranite 34 36 19 4 6 1 11 medium Aplitic/granophy

Granodiorite of Colorado Gulch Colorado Gulch Granodiorite 44 22 25 5 5 1 11 medium Porphyritic 

Monzogranite of Moosetown Moosetown Monzogranite 36 27 28 TR 8 1 9 medium Inequigranular to 

hypidiomorph

Monzogranite of Hell Canyon Hell Canyon Monzogranite 44 25 24 4 3 1 8 medium Inequigranular, hypidiomorph

Monzogranite of Pulpit Rock Pulpit Rock Monzogranite 31 33 28 7 1 8 medium Inequigranular, hypidiomorph

Monzogranite of Homestake Homestake Monzogranite 33 31 28 1 6 TR 7 medium Inequigranular, hypidiomorph

Granodiorite of Donald Donald Granodiorite 42 22 29 TR 5 1 6 medium Seriate, hypidiom

Monzogranite of Moose Creek Moose Creek Monzogranite 40 25 28 5 1 6 medium Porphyritic, hypidiomorph

Granophyre Granophyre Syenogranite 31 31 35 TR 2 TR 2 fine Aplitic, granophy

Granophyre of

Deer Creek

Deer Creek Monzogranite ~33 ~33 ~33 1 TR 1 fine Equigranular, apl

granophyric
Petrographic Data 7

Q Q

A P

5

20

60

10 35 65 90

Q Q

A P

5

20

60

10 35 65 90

Q Q

A P

5

20

60

10 35 65 90

Q Q

A P

5

20

60

10 35 65 90

Q Q

A P

5

20

60

10 35 65 90

Q Q

A P

5

20

60

10 35 65 90

Q Q

A P

5

20

60

10 35 65 90

Q Q

A P

5

20

60

10 35 65 90

Burton Park

CI=24

mafic

Butte

CI=13

intermediate

Climax Gulch

CI=12

intermediate

Donald

CI=6

intermediate

Camp

Thunderbird

CI=11

intermediate

Hell Canyon

CI=8

intermediate

Homestake

CI=7

intermediate

Moose Creek

CI=6

intermediate

Syenogranite Monzogranite Granodiorite

Quartz monzonite

Quartz

monzodiorite

Monzonite Monzodiorite

Tonalite

N:\Jeff\den12_cmrk00_0094_pp_dubray\report_figures\figure_03a.ai

Figure 3. Quartz (Q), alkali feldspar (A), plagioclase (P) ternary diagram showing modal compositions of Boulder batholith 

plutons. Diamond-shaped symbols represent modal compositions of individual samples and square-shaped symbols depict the 

computed average composition for each intrusion. Classification grid and rock names are those of Streckeisen (1976). Average 

color index (CI) and mafic, intermediate, or felsic designations are shown for each intrusion.
8 Synthesis of Petrographic, Geochemical, and Isotopic Data for the Boulder Batholith, Southwest Montana

Q Q

A P

5

20

60

10 35 65 90

Q Q

A P

5

20

60

10 35 65 90

Q Q

A P

5

20

60

10 35 65 90

Q Q

A P

5

20

60

10 35 65 90

Q Q

A P

5

20

60

10 35 65 90

Q Q

A P

5

20

60

10 35 65 90

Moosetown

CI=9

intermediate

Colorado Gulch

CI=11

intermediate

Pulpit Rock

CI=8

intermediate

Rader Creek

CI=17

mafic

Unionville

CI=18

mafic

Granophyre

CI=2

felsic

N:\Jeff\den12_cmrk00_0094_pp_dubray\report_figures\figure_03b.ai

Figure 3. Quartz (Q), alkali feldspar (A), plagioclase (P) ternary diagram showing modal compositions of Boulder batholith 

plutons. Diamond-shaped symbols represent modal compositions of individual samples and square-shaped symbols depict the 

computed average composition for each intrusion. Classification grid and rock names are those of Streckeisen (1976). Average 

color index (CI) and mafic, intermediate, or felsic designations are shown for each intrusion.—Continued
Major Oxide Data 9

Table 2. Average mineralogic and chemical 

compositions of Butte Granite north and south of latitude 

46.2°N; Boulder batholith, southwest Montana.

[n=number of analyzed samples; std dev, standard deviation; Plag, 

plagioclase; Alk fldsp, alkali feldspar, Qtz, quartz; Hb, hornblende; 

Bi, biotite; Opq, opaque oxide minerals; Acc, accessory minerals]

North (n=20) South (n=8)

Mean std dev Mean std dev

Plag 38.0 6.0 37.7 5.5

Alk fldsp 26.7 6.2 24.1 5.1

Qtz 21.2 5.2 23.9 4.0

Hb 5.2 3.2 5.7 2.5

Bi 6.6 2.6 7.5 2.0

Opq 1.6 0.7 1.3 0.8

Acc 0.6 0.2 0.5 0.7

North (n=49) South (n=37)

Mean std dev Mean std dev

SiO2 65.97 2.74 66.11 2.29

TiO2 0.57 0.13 0.56 0.12

Al2

O3 15.63 0.79 15.57 0.55

FeO* 4.35 1.21 4.46 0.89

MnO 0.10 0.08 0.09 0.02

MgO 2.05 0.83 2.08 0.49

CaO 3.74 1.06 3.99 0.61

Na2

O 3.14 0.46 2.92 0.16

K2

O 4.26 0.50 4.03 0.33

P2

O5 0.20 0.09 0.18 0.06

LOI 0.74 0.51 0.58 0.15

Major Oxide Data

Average chemical compositions of the Boulder batholith 

plutons are summarized in table 3. Major oxide compositions 

of the northern and southern parts of the Butte Granite, like 

relative mineral abundances, are statistically indistinguishable (table 2). The SiO2

 content of most Boulder batholith 

rocks ranges continuously from about 60 to 78 weight percent, 

although a few samples contain less than 60 weight percent 

SiO2. Samples of the Butte Granite, as well as those of the Climax Gulch granodiorite and Rader Creek Granodiorite, form 

broad compositional arrays that span as much as 13 weight 

percent SiO2. In contrast, smaller sample suites for peripheral plutons define more uniform compositional clusters. On 

geochemical variation diagrams (fig. 4), plutons of the Boulder batholith form relatively coherent composition clusters or 

linear arrays that are distinct but broadly overlapping. Specific 

plutons display both absolute and relative major oxide enrichments, or depletions relative to other plutons of the batholith. 

Absolute enrichments or depletions are by comparison to all 

Boulder batholith sample data, whereas relative enrichments 

or depletions indicate high or low concentrations at any given 

SiO2

 concentration. Distinctive major oxide features of the 

Boulder batholith plutons are as follows (fig. 4):

Mafic Group

Burton Park Granodiorite: elevated (absolute) TiO2, FeO*, 

MnO, MgO, CaO, P2O5 and low (absolute) Na2O

Unionville Granodiorite: elevated (absolute) TiO2, FeO*, 

MnO, MgO, CaO, P2O5 and low (absolute) Na2O

Rader Creek Granodiorite: elevated (relative) CaO, Na2O,

and Na/K and low (relative) TiO2 and K2O

Intermediate Group

Butte Granite: elevated (relative) TiO2, K2O, and MgO 

(slightly) and low (relative) Al2O3, Na2O, and Na/K

Climax Gulch granodiorite: no distinctive characteristics

Donald granodiorite: slightly elevated (absolute) SiO2 and 

Na2O and slightly elevated (relative) Al2O3, and FeO*

Camp Thunderbird monzogranite: slightly low (relative) 

FeO*, MgO, and CaO

Hell Canyon monzogranite: slightly elevated (relative) Al2O3, 

CaO, and Na2O

Homestake monzogranite: slightly elevated (relative) MgO 

and K2O and slightly low (relative) Al2O3 and Na2O

Moose Creek monzogranite: high (absolute) Na2O, high (relative) MnO and low (relative/absolute) FeO*, TiO2

, 

MgO, and CaO

Moosetown monzogranite: no distinctive characteristics

Colorado Gulch granodiorite: low (relative) MnO and CaO

Pulpit Rock monzogranite: slightly elevated (relative) P2O5

Felsic Group

Deer Creek granophyre: no distinctive characteristics

Granophyric intrusions: high (absolute) SiO2, and K2O and low 

(absolute) TiO2

, Al2O3, FeO*, MnO, MgO, and CaO

Tilling (1973) and Lambe (1981) showed that the chemistry of Boulder batholith rocks define two distinct magma 

composition groups, which were identified as the main and 

sodic magma series, respectively. Relative to major oxide 

data, plutons of the sodic series (Rader Creek Granodiorite, 

and the Hell Canyon monzogranite, Donald granodiorite, 

Climax Gulch granodiorite, Moose Creek monzogranite, 

and Moosetown monzogranite) were defined as those having 

higher Na2

O and lower K2O contents than plutons of main 

series (Butte Granite, Unionville Granodiorite, and granophyric intrusions); assignment of the Burton Park granodiorite, the Camp Thunderbird monzogranite, and the Colorado 

Gulch granodiorite to the appropriate magma series was 

uncertain (Tilling, 1973). Data compiled by du Bray and others (2009) affirm that, at any given silica content, the Rader 

Creek Granodiorite is more sodic than other Boulder batholith 
Table 3. Average chemical compositions for plutons of the Boulder batholith, southwest Montana, arranged in order of increasing silica content. [n=, number of samples upon which major oxide averages are based. Major oxides, in weight percent, recalculated to 100 percent, volatile free. F and Cl abundances in weight pdances in parts per million. Blank fields constitute no data] Burton Park Union- ville Rader Creek Camp Thunder- bird Butte Colorado Gulch Climax Gulch Home- stake Pulpit Rock Hell Canyon Donald Moose- town Mo

Cr

n= 9 9 100 6 86 4 43 4 2 13 29 3 

SiO2 61.35 61.60 65.13 65.32 66.02 68.72 68.44 69.49 69.56 69.74 71.46 72.24 7

TiO2 0.74 0.71 0.47 0.55 0.57 0.41 0.43 0.42 0.36 0.33 0.29 0.23 Al2O3 15.84 15.30 16.13 16.66 15.59 15.55 15.53 14.75 14.98 15.80 14.84 15.16 1FeO* 6.79 6.08 4.68 4.18 4.40 3.12 3.41 2.91 3.09 2.76 2.76 1.92 

MnO 0.14 0.12 0.11 0.11 0.10 0.07 0.08 0.08 0.08 0.09 0.07 0.07 

MgO 3.23 3.35 1.99 1.74 2.06 1.06 1.55 1.68 1.18 0.95 0.84 0.71 

CaO 5.57 5.86 4.60 3.90 3.85 2.84 3.39 3.22 3.13 3.17 2.54 2.10 

Na2O 2.88 2.86 3.65 3.51 3.05 3.46 3.19 2.87 3.13 3.62 3.65 3.70 

K2O 3.21 3.81 3.01 3.76 4.16 4.58 3.81 4.42 4.25 3.39 3.39 3.77 

P2O5 0.25 0.31 0.23 0.27 0.20 0.19 0.17 0.16 0.24 0.15 0.16 0.10 

vol sum 0.76 0.77 0.64 0.77 0.90 0.79 0.44 0.80 0.76 0.63 0.48 0.39 

F 0.06 0.10 0.05 0.05 

Cl 0.02 0.02 0.01 0.01 

Ba 1,101 1,002 746 792 845 609 1,212 1,197 750 1,16

Rb 184 126 148 297 171 173 245 118 15

Sr 745 601 426 538 371 543 527 387 28

Th 11.6 20.6 21.7 13.0 18.3 54.3 23.1 44.3 34.5 21.3 2

U 3.50 5.04 2.24 2.70 4.57 5.80 5.29 3.41 3.28 

Co 29 8 22 30 100 25 1Ni 45 33 30 27 16 18 15 1

V 63 74 85 69 39 5
Major Oxide Data 11

N:\Jeff\den12_cmrk00_0094_pp_dubray\report_figures\figure_04.ai

SiO2, in weight percent

SiO2

, in weight percent

0.0

0.1

0.2

0.3

0.4

0.5

Granophyre

Burton Park

Butte

Deer Creek

Donald

Hell Canyon

Homestake

Moose Creek

Moosetown

Pulpit Rock

Rader Creek

Unionville

Climax Gulch

Camp Thunderbird

Colorado Gulch

TiO2, in weight percent Al O2 3, in weight percent FeO*, in weight percent MnO, in weight percent MgO, in weight percent

Cao, in weight percent Na O, in weight percent

2 K O, in weight percent

2 P O2 5, in weight percent

EXPLANATION

[Pluton short name]

0.1

0.2

0.0

0

1

2

3

4

5

6

7

8

9

2

3

4

5

1

2

3

4

5

6

0

1

2

3

4

5

55 60 65 70 75

55 60 65 70 75

0.0

0.2

0.4

0.6

0.8

1.0

Sodic series

Sodic series

Main series

Main series

Shoshonitic

High-K

Low-K

Medium-K

11

12

13

14

15

16

17

18

19

0

1

2

3

4

5

6

7

8

9

Figure 4. Variation diagrams showing major oxide abundances, in weight percent, in samples of the Boulder batholith, southwest 

Montana. Black field boundaries on K2O versus SiO2 diagram from Le Maitre and others (1989); high K-shoshonite dividing line from Ewart 

(1982). Red field boundaries on K2

O versus SiO2 and Na2O versus SiO2 diagrams from Tilling (1973).
12 Synthesis of Petrographic, Geochemical, and Isotopic Data for the Boulder Batholith, Southwest Montana

intrusions, and indicate, in particular, that Na/K ratios for 

Rader Creek Granodiorite samples are distinctly elevated 

relative to all other batholith samples. Other plutons included 

in Tilling’s sodic series contain more Na2O and less K2O than 

main series intrusions but simply define compositional continua from main series intrusion compositions toward those 

characteristic of the compositionally distinct Rader Creek 

Granodiorite. Na2O and K2O contents of the Rader Creek 

Granodiorite are distinct relative to those of other Boulder 

batholith rocks but the existence of separate sodic and main 

magma series within the batholith is not convincingly demonstrated by available major oxide data.

Among the Boulder batholith rocks, concentrations of 

TiO2, Al2O3, FeO*, MnO, MgO, CaO, and P2O5 decrease 

systematically with increasing SiO2, whereas K2O contents 

increase with increasing SiO2; Na2O contents display no 

consistent relation with SiO2 content. Average Na/K is 1.0 

but ranges from 0.4 to 1.7; as a group, samples of the Rader 

Creek Granodiorite are distinctly sodic whereas samples of 

the Butte Granite are distinctly potassic. Relative amounts 

of K2O and SiO2 in most Boulder batholith rocks (fig. 4) are 

coincident with high- to medium-K compositions defined by 

Gill (1981) and Le Maitre (1989).

Volatile contents of the Boulder batholith rocks are 

generally similar to those characteristic of granitoid rocks. 

The majority of the Boulder batholith rocks contain less than 

700 parts per million (ppm) F, which is consistent with average (500–900 ppm) F contents of granitoid rocks (Turekian 

and Wedepohl, 1961). However, a group of about 20 samples 

of the Climax Gulch granodiorite contain distinctly elevated 

F abundances that range from about 900 ppm to almost 

1,700 ppm. Average Cl contents of granitoid rocks range 

from about 100 to 200 ppm (Turekian and Wedepohl, 1961), 

and most Boulder batholith rocks have Cl abundances in 

this range. A few samples of the Rader Creek Granodiorite, 

Climax Gulch and Donald granodiorites, and Moose Creek 

and Hell Canyon monzogranites have Cl abundances in the 

200–550 ppm range.

Major oxide compositions of the Boulder batholith are 

similar to those of subduction-related igneous rocks. Relative 

to standard metrics, they are subalkaline (Irvine and Baragar, 

1971), magnesian (Frost and others, 2001), calc-alkalic to 

calcic (Frost and others, 2001; Miyashiro, 1974), and metaluminous to weakly peraluminous (Shand, 1951). On variation 

diagrams (figs. 5–8) associated with these metrics, each of the 

Boulder batholith plutons forms a relatively coherent compositional cluster or array; however, compositions of the various 

plutons are broadly overlapping.

Trace Element Data

Trace element abundances (du Bray and others, 2009) 

for the plutons of the Boulder batholith form clusters that are 

broadly diagnostic of their constituent plutons. The Climax Gulch and Donald granodiorites and the Hell Canyon 

monzogranite are distinctly enriched in incompatible trace elements (table 3). Distinctive trace element features (absolute, 

unless otherwise indicated) of the Boulder batholith plutons 

are as follows:

Mafic Group

Burton Park granodiorite: elevated Cu, Ni, Pb (relative), Sc, Y, 

Zn, Zr, and low Th

Unionville Granodiorite: elevated Ni, Sc, U, Zn, and low Sr

Rader Creek Granodiorite: elevated Ba, Ni, Sr, low U, and low 

V and Th (relative)

Intermediate Group

Butte Granite: elevated Cu, Sc, U, Y (relative), Zr, Zn, low Ba, 

Pb, and Sr

Climax Gulch granodiorite: elevated F, Rb, Th, V, Zn, and 

low Cu

Donald granodiorite: elevated Ba, Pb, Rb, Th, and low Ni 

and V

Camp Thunderbird monzogranite: elevated Zn

Hell Canyon monzogranite: elevated Co, Mo, and Th

Homestake monzogranite: low Ba

Moose Creek monzogranite: low Ni and Sr

Moosetown monzogranite: elevated Zn

Colorado Gulch granodiorite: no distinctive characteristics

Pulpit Rock monzogranite: low Co and Pb

Felsic Group

Deer Creek granophyre: low Co, Cr, Sc, and Zn

Granophyric intrusions: low Sr and Zn

Large-ion lithophile element (LILE) and high-fieldstrength element (HFSE) abundances for Boulder batholith 

samples are similar to those of convergent-margin, broadly 

calc-alkaline, subduction-related igneous rocks such as in the 

Andean, Kamchatka, and Central American arcs (GEOROC, 

2007). Tectonic classification schemes based on relative 

trace element abundances, including Rb versus Y + Nb of 

Pearce and others (1984), are consistent with plutons of the 

Boulder batholith having an origin in a subduction-related 

magmatic arc environment (fig. 9). Magma genesis in this 

type of tectonic setting is further indicated by incompatible trace element ratios. Wood and others (1979) and Gill 

(1981) established that Ba/Ta, Ba/Nb, and La/Nb ratios for 

modern arc rocks are greater than 450, greater than 26, and 

2–7, respectively. Average Ba/Ta, Ba/Nb, and La/Nb for all 

Boulder batholith samples are 793, 50, and 2.4, respectively 

(du Bray and others, 2009). Insufficient trace element data are 

available to determine whether individual Boulder batholith 

plutons have distinctive ranges of values for each of these 

trace element parameters.

Relative abundances of K, Rb, and Sr in samples of 

most Boulder batholith plutons form broadly diagnostic 
Trace Element Data 13

4

5

6

7

8

9

10

55 60 65 70 75

Alkaline

Subalkaline

Na O+K 2 O, in weight percent

2

SiO2, in weight percent

Granophyre

Burton Park

Butte

Deer Creek

Donald

Hell Canyon

Homestake

Moose Creek

Moosetown

Pulpit Rock

Rader Creek

Climax Gulch Unionville

Camp Thunderbird

Colorado Gulch

[Pluton short name]

EXPLANATION

N:\Jeff\den12_cmrk00_0094_pp_dubray\report_figures\figure_05.ai

Figure 5. Total alkali-silica variation diagram showing compositions of the Boulder batholith, southwest Montana. Alkaline and 

subalkaline fields defined by Irvine and Baragar (1971).

compositional clusters (fig. 10) and have relatively Sr-enriched 

compositions; Rader Creek Granodiorite samples, in particular, are strongly Sr-enriched (fig. 10). Samples of the Moose 

Creek and Homestake monzogranites and of granophyre have 

progressively K- and Rb-enriched compositions; a sample of 

the Deer Creek granophyre has the most evolved composition 

of any Boulder batholith sample.

As summarized by John and others (2010), genesis 

of porphyry copper deposits has been linked with adakites, 

magmatic arc rocks with unique geochemical compositions 

whose genesis may have involved components derived from 

subducted slab partial melting. Richards and Kerrich (2007) 

summarize the geochemistry of these rocks and suggest that 

among other characteristics, Sr/Y greater than or equal to 20, 

Y less than or equal to 18 ppm, SiO2 greater than or equal to 

56 percent, Yb less than or equal to 1.9 ppm, Ni greater than 

or equal to 20 ppm, Cr greater than or equal to 30 ppm, and 

La/Yb greater than or equal to 20 are diagnostic of adakites. 

Most of the analyzed Boulder batholith rocks do have Sr/Y 

greater than or equal to 20 and SiO2 greater than or equal to 

56 percent, but data for the few Boulder batholith samples 

for which La/Yb and abundances of Y, Yb, Ni, and Cr have 

been determined are only marginally similar to those diagnostic of adakites.
14 Synthesis of Petrographic, Geochemical, and Isotopic Data for the Boulder Batholith, Southwest Montana

Figure 6. FeO*/(FeO*+MgO) versus SiO2 variation diagram showing the compositions of the Boulder batholith, southwest Montana. 

FeO*, total iron expressed as ferrous iron. Ferroan versus magnesian boundary (black line) from Frost and others (2001); tholeiitic versus 

calc-alkaline boundary (red line) from Miyashiro (1974).

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

55 60 65 70 75

FeO*/(FeO*+MgO), in weight percent

SiO2, in weight percent

ferroan

magnesian

Tholeiitic

Calc-alkaline

Granophyre

Burton Park

Butte

Deer Creek

Donald

Hell Canyon

Homestake

Moose Creek

Moosetown

Pulpit Rock

Rader Creek

Climax Gulch Unionville

Camp Thunderbird

Colorado Gulch

[Pluton short name]

EXPLANATION

N:\Jeff\den12_cmrk00_0094_pp_dubray\report_figures\figure_06.ai

Abundances of most trace elements in Boulder batholith rocks are only weakly correlated with SiO2

 abundance 

variations. Among all Boulder batholith samples for which 

trace element data are available, Cr, Cu, Ni, Sc, Zn, and Zr 

contents decrease (somewhat inconsistently for some of 

these elements) with increasing SiO2

, whereas Pb concentrations increase somewhat inconsistently with increasing SiO2

content (du Bray and others, 2009). The lack of additional 

and better correlated relations between trace element concentrations and variable silica content is uncharacteristic of 

igneous rock geochemical variation patterns and shows a 

lack of accuracy and(or) precision of trace element data for 

samples (analyzed by a variety of techniques) included in 

the Boulder batholith geochemical compilation. However, 

the compilation of du Bray and others (2009) includes high 

quality data for a group of 23 samples analyzed by modern 

techniques (with good associated accuracy and precision). 

These samples include five samples with “02HW” as first four 

characters of their sample designation, 17 samples with “KL” 

as the third and fourth characters of their sample designation, 
Trace Element Data 15

Figure 7. Na2O+K2O-CaO versus SiO2 variation diagram showing the compositions of the Boulder batholith, southwest Montana. 

Boundaries between various rock series from Frost and others (2001).

N:\Jeff\den12_cmrk00_0094_pp_dubray\report_figures\figure_07.ai

alkalic

calc-alkalic

alkali-calcic

calcic

−8

−6

−4

0

4

6

8

12

55 60 65 70 75

Na O+K 2 O-CaO, in weight percent

2

SiO2, in weight percent

2

−2

10

Granophyre

Burton Park

Butte

Deer Creek

Donald

Hell Canyon

Homestake

Moose Creek

Moosetown

Pulpit Rock

Rader Creek

Climax Gulch Unionville

Camp Thunderbird

Colorado Gulch

[Pluton short name]

EXPLANATION

and the sample with the identifier “Macdonald.” Samples 

in this subset of the compilation have Ba, Pb, Nb, and Rb 

contents that increase with SiO2 and contents of Cs, Co, Cu, 

Cr, Ga, Sc, V, Y, Zn, and Zr that decrease with increasing 

SiO2. With the exception of Co, Ga, Nb, Sc, Th, V, and Zn, 

these covariation arrays are somewhat inconsistent. Even 

among more well-correlated abundance variations, many of 

the concentration increases or decreases are of moderate relative magnitude ([highest concentration-lowest concentration]/

highest concentration ): Ba (+56 percent), Pb (+67 percent), 

Nb (+63 percent), and Rb (+64 percent); Cs (–60 percent), 

Co (–95 percent), Cu (–81 percent), Cr (–96 percent), 

Ga (–28 percent), Sc (–82 percent), V (–89 percent), Y 

(–62 percent), Zn (–76 percent), and Zr (–86 percent).

The limited amount of ore metal abundance data (specifically for Cu, Mo, Pb, and Zn) available for unmineralized 

Boulder batholith samples (du Bray and others, 2009) are comparable to ore metal global average abundances (Turekian and 

Wedepohl, 1961) in basalt (100, 1, 5, and 100 parts per million, 

respectively) and granite (10, 2, 20, and 40 parts per million, 
16 Synthesis of Petrographic, Geochemical, and Isotopic Data for the Boulder Batholith, Southwest Montana

Figure 8. Variation diagram showing molar major oxide compositions the Boulder batholith, southwest Montana as a function of 

alumina and alkali saturation. Al O2 3/(Na O+K 2 O)

2

Metaluminous Peraluminous

Strongly peraluminous

Peralkaline

N:\Jeff\den12_cmrk00_0094_pp_dubray\report_figures\figure_08.ai

0.9

1.1

1.3

1.7

2.1

2.3

2.5

2.7

0.6 0.8 1.0 1.2

1.9

1.5

0.7 0.9 1.1 1.3 1.4 1.5

Al2O3/(CaO+Na2O+K2O)

Granophyre

Burton Park

Butte

Deer Creek

Donald

Hell Canyon

Homestake

Moose Creek

Moosetown

Pulpit Rock

Rader Creek

Climax Gulch Unionville

Camp Thunderbird

Colorado Gulch

[Pluton short name]

EXPLANATION

respectively). Copper concentrations in Boulder batholith rocks 

range from several parts per million (ppm) to about 45 ppm 

(average approximately 17 ppm). Molybdenum concentrations range from less than 1 to 20 ppm (average approximately 

8 ppm). Lead concentrations range from about 4 to 36 ppm 

(average approximately 21 ppm). Zinc concentrations range 

from about 7 to 69 ppm (average approximately 61 ppm). 

Consequently, with the exception of Mo, abundances of the Cu, 

Mo, Pb, and Zn in Boulder batholith rocks are not significantly 

elevated relative to ordinary igneous rocks. The majority of the 

Mo concentrations available for Boulder batholith rocks reflect 

analyses conducted more than 30 years ago by undocumented 

methods (du Bray and others, 2009) and are of uncertain accuracy and precision. With two exceptions, the group of about 

23 recent, high quality analyses (described in the preceding 

paragraph) contain less than 2 ppm Mo; one additional sample 

contains 3 ppm Mo and a sample bearing visible molybdenite 

in miarolitic cavities (Deer Creek granophyre) contains 156 

ppm Mo. Consequently, although polymetallic quartz vein 

deposits are approximately coeval with the younger plutons 
U-Pb Zircon Geochronology 17

and are hosted by the batholith (Lund, 2007; Lund and others, 

2002), unmineralized Boulder batholith magmas do not contain 

elevated abundances of the metals (Cu, Mo, Pb, and Zn) that 

are significant constituents of those deposits.

Accurate and precise rare earth element (REE) data 

available for Boulder batholith samples are restricted to 

the representative set of 23 samples (described two paragraphs previously). Chondrite-normalized REE patterns for 

Boulder batholith rocks (fig. 11) are typical of intermediate 

composition, calc-alkaline subduction-related igneous rocks 

(for example, Cameron and Cameron, 1985; Feeley and 

Davidson, 1994; Gill, 1981; Wark, 1991). Boulder batholith 

REE patterns have negative slopes and most have insignificant to small negative europium anomalies although the 

Rader Creek Granodiorite (Eu/Eu* = 1.09), Donald granodiorite (Eu/Eu* = 1.18), and Hell Canyon monzogranite 

(Eu/Eu* = 1.28) have small, unusual positive Eu anomalies 

consistent with plagioclase accumulation. Eu/Eu* ranges from 

0.69 to 1.28 (average, 0.87) and the magnitude of Eu anomalies is unrelated to SiO2

 content. Among Boulder batholith 

samples, those with the highest SiO2

 content have slightly 

low total REE contents and slightly clockwise-rotated REE 

patterns (relatively higher light [LREE] and lower heavy REE 

[HREE] concentrations). Overall, LREE pattern segments are 

somewhat more steeply sloping than HREE segments, which 

are flat to slightly U-shaped.

With several exceptions, REE abundances (fig. 11) in 

Boulder batholith pluton samples vary within a relatively 

restricted range, yield calculated negative europium anomalies of similar magnitude, and define REE patterns that are 

essentially parallel (LaN/YbN values vary minimally). In 

particular, REE abundance patterns for seven Butte Granite 

samples (representative of the full Butte Granite age array) are 

indistinguishable within the limits of analytical uncertainty. 

Presuming that these samples are representative of the entire 

Butte Granite pluton, they show that the composition of this 

large intrusive is remarkably homogeneous. In contrast, the 

LREE content of the Homestake monzogranite is low relative to other Boulder batholith rocks. REE patterns for the 

Donald granodiorite, Hell Canyon monzogranite, and Deer 

Creek granophyre are somewhat distinctive. All three plutons 

are distinguished by atypically low HREE abundances; HREE 

abundances for the Homestake monzogranite are similarly 

low. These samples also have the three lowest Eu contents 

among the Boulder batholith samples; Eu for the Deer Creek 

sample, in particular, is dramatically lower than that of any 

other Boulder batholith rock. Finally, the Deer Creek granophyre and Hell Canyon monzogranite, and to a lesser extent, 

the Donald granodiorite have unusually low LREE contents 

that are distinct relative to all other Boulder batholith rocks.

Primitive mantle-normalized trace element patterns for 

Boulder batholith plutons are gently negatively sloping, having high large-ion lithophile element (LILE) abundances and 

low high-field-strength element (HFSE) abundances common 

among moderately differentiated subduction-related magmas 

(fig. 12). These patterns depict systematic but relatively 

limited geochemical variation among Boulder batholith plutons; patterns for seven Butte Granite samples are essentially 

parallel, clustered, and represent consistent reservoir-wide 

petrogenetic processes. Large negative Nb-Ta anomalies, similar to those characteristic of subduction-related magmatism 

(Wood and others, 1979; Gill, 1981; Pearce and others, 1984), 

are also characteristic of Boulder batholith samples.

The three most mafic Boulder batholith plutons and all 

but the Deer Creek granophyre and Hell Canyon monzogranite have primitive mantle-normalized patterns that are 

essentially indistinguishable. The Deer Creek granophyre 

and Hell Canyon monzogranite define distinctive primitive 

mantle-normalized patterns that have almost uniformly low 

trace element (and as described above, particularly REE) 

abundances; the Homestake monzogranite exhibits some of 

these same characteristics.

Tilling (1973) and Lambe (1981) showed that Sr 

abundances are high and Rb abundances low in sodic series 

rocks relative to main series rocks; Tilling (1973) and Lambe 

(1981) also show that concentrations of Ba, Nb, Cu, Th, 

and U are distinct for samples of the two magma series. 

The compilation of du Bray and others (2009) indicates that 

among the well-analyzed Boulder batholith samples, Sr and 

Ba concentrations are distinctly elevated in Rader Creek 

Granodiorite samples whereas Rb, Th, and U contents are 

low relative to other Boulder batholith rocks. In addition, Y, 

Cs, Zr, Hf, and REE (especially HREE) contents are slightly 

lower in the Tilling (1973) sodic series rocks than in main 

series rocks. However, the geochemistry of plutons previously interpreted as part of the sodic series is otherwise not 

statistically distinct relative to that of other Boulder batholith 

rocks. As earlier inferred from major oxide data, the Rader 

Creek Granodiorite has some distinctive compositional 

attributes but trace element data do not indicate a definitive 

geochemical distinction between plutons previously designated as constituents of the sodic and main magma series.

U-Pb Zircon Geochronology

Previously, Lund and others (2002) published ages of 

four plutons (Rader Creek, Unionville, Butte Granite, and 

Pulpit Rock) of the Boulder batholith. This study presents 

new data and ages for an additional 12 plutons and a sample 

of granophyre, all associated with the batholith (fig. 13, 

table 4). In addition, zircons from each of the previously 

dated plutons (Lund and others, 2002) were reanalyzed to 

produce an internally consistent dataset.

Methods

Rock samples were collected from outcrops throughout the Boulder batholith and zircons were extracted from 

these samples using standard mineral separation techniques 

of crushing, pulverizing, Wilfley table, Frantz magnetic 
18 Synthesis of Petrographic, Geochemical, and Isotopic Data for the Boulder Batholith, Southwest Montana

Within plate

Ocean ridge Volcanic arc

Syn-collisional

Y + Nb, in parts per million

Rb, in parts per million

1,000

100

10

1

1 10 100 1,000

N:\Jeff\den12_cmrk00_0094_pp_dubray\report_figures\figure_09.ai

Granophyre

Burton Park

Butte

Deer Creek

Donald

Hell Canyon

Homestake

Moose Creek

Moosetown

Pulpit Rock

Rader Creek

Climax Gulch Unionville

Camp Thunderbird

Colorado Gulch

[Pluton short name]

EXPLANATION

Figure 9. Trace-element, tectonic-setting-discrimination variation diagram showing the composition 

of samples of the Boulder batholith and its peripheral plutons, southwest Montana. Tectonic settingcomposition boundaries from Pearce and others (1984).

separator, and Methylene Iodide. Grains were handpicked 

onto double-stick tape, cast in epoxy, ground to about half 

thickness, and polished with 6 micrometers (μm) and 1μm 

diamond suspension.

All grains were imaged in reflected and transmitted light 

on a petrographic microscope and in cathodoluminescence 

(CL) on the USGS-Denver JEOL 5800LV scanning electron 

microscope (fig. 13). These images were used as guides for the 

location of ion microprobe analysis spots. U-Pb geochronologic analyses (table 4) were made using the USGS/Stanford 

University sensitive high resolution ion microprobe-reverse 

geometry (SHRIMP-RG) instrument. The primary ion beam 

was focused to extract material from a pit about 25–30 μm in 

diameter. Mass stations were cycled through five iterations per 

analysis. 206Pb/238U ratios were calibrated against zircon standard R33 (419 Ma; Black and others, 2004). Raw data were 

reduced using SQUID (Ludwig, 2001); results are shown on 

Tera-Wasserburg concordia plots and weighted averages plots 

created using Isoplot 3 (Ludwig, 2003). Concordia plots were 

used to assess which analyses should be included in the age 

calculations; the weighted average of selected 206Pb/238U ages 

was used to determine the age of each sample (fig. 13).

The primary focus of the geochronologic analyses was to 

determine Boulder batholith pluton crystallization ages; zircon 

grains and spots selected for analysis were chosen accordingly. 

Only pristine (and in most cases, homogeneous) grains were 

selected for SHRIMP analysis. Results for all dated plutons 

are summarized in table 5.

Results

All sampled plutons yielded elongate, colorless, euhedral 

zircons with various degrees of oscillatory zoning indicative of 

crystallization in a magma. Most of these zircons have modest 

U concentrations (about 100–500 ppm) and low common Pb 

content. All of the dated grains from the plutons yielded concordant U-Pb data. In contrast, zircons from sample 03KL071 

(granophyre of Deer Creek) are dark to opaque, equant to 
U-Pb Zircon Geochronology 19

N:\Jeff\den12_cmrk00_0094_pp_dubray\report_figures\figure_10.ai

K/100

Rb Sr

Granophyre

Burton Park

Butte

Deer Creek

Donald

Hell Canyon

Homestake

Moose Creek

Moosetown

Pulpit Rock

Rader Creek

Climax Gulch Unionville

Camp Thunderbird

Colorado Gulch

[Pluton short name]

EXPLANATION

Figure 10. Variation diagram showing relative proportions of rubidium, potassium, and strontium 

in samples of the Boulder batholith and its peripheral plutons, southwest Montana.

stubby, show little or no oscillatory zoning (fig. 13), and are 

speckled with numerous small, irregularly distributed domains 

that are bright in CL images. All grains from this sample have 

high U (2,000–6,000) and common Pb contents (table 4). 

Although the six oldest grains indicate an age of about 78 Ma, 

four other grains are discordant (table 4). Thus, this study cannot determine a reliable age for the granophyre of Deer Creek.

SHRIMP U-Pb geochronology of zircon shows that 

plutons of the Boulder batholith were emplaced and solidified over a period of 6–10 m.y. (81.7 ± 1.4 to 73.7 ± 0.6 Ma; 

table 5). Zircons in the Rader Creek Granodiorite contain 

abundant Archean and Paleoproterozoic inheritance in 

rounded cores (Lund and others, 2002). In all other plutons, 

inheritance seems to be minimal, although the apparent lack 

of older material may reflect sampling bias. Alternatively, 

Boulder batholith plutons that lack inherited zircon may 

have followed different emplacement and temperature paths 

that either limited assimilation of inherited zircon or, having 

incorporated exotic zircon grains, caused them to be completely dissolved.

U-Pb zircon ages of most Boulder batholith plutons are 

correlated with intrusion composition. The three most mafic 

plutons (Rader Creek, Unionville, and Burton Park), whose 

ages range from 80.7 to 77.3 Ma, are among the oldest components of the batholith. The voluminous, intermediate composition Butte Granite was the next phase of the batholith to be 

emplaced (about 76.7 Ma); a second apparent pulse of Butte 

Granite magmatism may have occurred at about 74.7 Ma. The 

progressively more felsic peripheral plutons were emplaced at 

about 76.7 to 73.7 Ma (table 5).
20 Synthesis of Petrographic, Geochemical, and Isotopic Data for the Boulder Batholith, Southwest Montana

Radiogenic Isotope Data

Neodymium and strontium isotopic data were obtained 

for 12 representative Boulder batholith samples (table 6). 

Initial 87Sr/86Sr (Sri) values for most of these samples cluster 

between about 0.707 and 0.709 and are similar to those 

reported for Boulder batholith rocks by Doe and others 

(1968) and Foster and others (2006) and unpublished data 

described by Taylor and others (2007). εNd values for these 

samples are variable, ranging from –11.9 to –22.9. Neither 

εNd nor Sri variations are well correlated with age, although 

the two youngest Boulder batholith samples (Moose Creek 

and Hell Canyon monzogranites) have distinctly more 

radiogenic Sri values. Three samples of Butte Granite and 

one sample of Pulpit Rock monzogranite, have the highest εNd values (–11.9 to –12.1) of the analyzed Boulder 

batholith samples; these samples define a small range of 

Sri

 values (0.70685 to 0.70726) that is comparable to most 

other Boulder batholith samples. Data for the Rader Creek 

Granodiorite, Donald granodiorite, and Homestake monzogranite define a second cluster having the lowest εNd values 

(–18.5 to –22.9); Sri data for these samples define a narrow 

range from 0.70695 to 0.70702. Five plutons (Unionville 

Granodiorite, Burton Park and Climax Gulch granodiorites, 

and Moose Creek and Hell Canyon monzogranites) yielded 

intermediate εNd values (–14.1 to –16.4) and more radiogenic 

Sri values (0.70716 to 0.70892).

As shown by Taylor and others (2007), strongly negative εNd values and relatively high Sri

 values for all analyzed (table 6) samples of the Boulder batholith indicate 

that these rocks were derived from Precambrian crust, an 

interpretation supported by ubiquitous 2.4–3.4 Ga cores in 

zircons from the Rader Creek Granodiorite (Lund and others, 2002). Samples from other plutons include little or no 

inheritance; however, because the focus of geochronologic 

results reported here was determination of emplacement 

ages, not source ages, the apparent lack of inheritance 

may represent a sampling issue. Sr and Nd isotopic data 

for Boulder batholith rocks are more radiogenic than those 

for plutons from accreted terranes in western Idaho and 

eastern Oregon, from the Salmon River suture, and from 

most of the Big Belt Mountains plutonic rocks (figs. 1 

and 14; Fleck and Criss, 1985; 2004; Fleck, 1990; Gunn, 

1991; Lund, 2007; Unruh and others, 2008). Sri

 values for 

the Boulder batholith are significantly less radiogenic than 

those for the Late Cretaceous Idaho batholith (to the west) 

but ε Nd values are more radiogenic; whereas, the converse 

is true relative to Late Cretaceous Pioneer batholith rocks 

(to the southwest). Sri values for the Boulder batholith are 

most similar to Eocene plutons on the eastern side of the 

Idaho batholith (Fleck and Criss, 1985; 2004).

Petrogenetic Implications of 

Geochemical and Isotopic Data

As documented above in the U-Pb zircon geochronology section, the age of the Butte Granite spans about 2 m.y. 

Consequently, either (1) the Butte Granite consists of 2 or 

more intrusive phases that solidified at different times or (2) 

U-Pb zircon ages of the Butte Granite depict a solidification 

process that spanned 2 m.y. Petrographic and geochemical 

characteristics of the northern and southern parts of the Butte 

Granite are indistinguishable. Given the diversity of petrographic and geochemical features characteristic of adjacent 

plutons in batholithic terranes (Bateman, 1992), it is unlikely 

that the Butte Granite is a composite mass composed of two 

fortuitously identical intrusions with regard to petrographic 

and geochemical characteristics. Consequently, the second 

hypothesis, that the Butte Granite solidified during 2 m.y., 

seems most consistent with geochronologic, petrographic, and 

geochemical attributes of these rocks.

Geochemical and isotopic data for the Boulder batholith 

rocks are consistent with their genesis in a subduction-related 

tectonic setting. Although not particularly diagnostic, major 

oxide abundances are similar to those of other continental 

magmatic arc rocks (du Bray and others, 1995; 2006; 2009). 

Trace element abundances are significantly more definitive 

in identifying the Boulder batholith magma as subduction 

related. In particular, relative abundances of Rb versus Y + Nb, 

Ba/Nb, Ba/Ta, La/Nb, and well-developed negative Nb-Ta 

anomalies show that all of the Boulder batholith rocks are the 

products of magmatic arc, subduction-related magmatism. In 

addition, primitive mantle-normalized trace element diagrams 

for Boulder batholith plutons include variably developed small 

negative Ba anomalies, prominent positive Pb anomalies and 

small to moderate negative P and Ti anomalies: each of these 

are similar to anomalies characteristic of intermediate composition, subduction-related, MASH-zone (Hildreth and Moorbath (1988) magmas that evolved by separation from restite 

during partial melting and(or) moderate fractionation of early 

crystallized minerals.

Down-going slab dehydration and attendant volatileinduced flux melting of lithospheric mantle beneath the 

western edge of the North American continent likely fostered 

the ascent of relatively mafic magmas into the lower crust. On 

the basis of radiogenic isotopic data, Foster and others (2006) 

suggest that the Paleoproterozoic basement, which underlies 

most of the Boulder batholith, was a fertile crustal source for 

partial melt generation. Mantle-derived, subduction-related 

partial melts may themselves have initiated additional partial 

melting of material included in the basement terrane, and 

mixed with those magmas to produce more voluminous, felsic 
Petrogenetic Implications of Geochemical and Isotopic Data 21

N:\Jeff\den12_cmrk00_0094_pp_dubray\report_figures\figure_11.ai

10

100

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Tm Yb Lu

EXPLANATION

04KL057 Burton Park

98KL003 Butte

98KL001a Butte

03KL067 Butte

98KL004 Butte

03KL068 Butte

02HW-28 Butte

02HW-30 Butte

Macdonald Camp Thunderbird

10KL072 Camp Thunderbird

03KL072 Climax Gulch

10KL073 Colorado Gulch

03KL071 Deer Creek

02HW-32 Deer Creek

04KL058 Donald

04KL058 Hell Canyon

03KL066 Homestake

04KL059 Moose Creek

98KL007 Pulpit Rock

98KL002 Rader Creek

02HW-31b Rader Creek

98KL005 Unionville

Sample/Chondrite

Rare earth element

Figure 11. Chondrite-normalized rare-earth element diagrams for Boulder batholith samples; chondrite abundances from Anders and 

Ebiharra (1982).

magmas such as those represented by the Boulder batholith. 

The presence of magmatic arc rocks well inboard from the 

western, subduction-zone delimited margin of western North 

America in the Late Cretaceous is consistent with low angle 

subduction (Coney and Reynolds, 1977; Lipman, 1992) and 

inboard deflection of the critical depth required for genesis of 

subduction-related magmas. Subsequently, as the convergence 

rate decreased, the subduction zone steepened and the hingeline of the down-going oceanic slab (and subduction-related 

magmatism) retreated westward (Coney and Reynolds, 1977; 

Lipman, 1992).

Distinct isotopic compositions within constituent plutons 

of the Boulder batholith probably represent different proportions of mafic, mantle-derived magma and crustally-derived 

partial melts. Sri

 and εNd isotopic variation preserved in the 

Boulder batholith rocks may also indicate isotopic inhomogeneities in the underlying continental basement that mixed with 

upwardly buoyant, mantle-derived mafic magma. Magmatism 

responsible for the Climax Gulch granodiorite and the Moose 

Creek and Hell Canyon monzogranites involved a component 

that was elevated with respect to radiogenic Sr, whereas that 

responsible for Butte granite and Pulpit Rock monzogranite 

was more primitive with regard to εNd than other Boulder 

batholith rocks and that responsible for the Donald granodiorite involved a source with elevated εNd.

Trace element abundances of Boulder batholith rocks 

show that the associated magmas experienced limited differentiation. In evolving magmatic systems affected by plagioclase separation, melt compositions become relatively Sr 

depleted and K enriched, and in the case of advanced fractionation, Rb enriched, with differentiation (Hanson, 1980). 

Relative abundances of K, Rb, and Sr in these rocks are 

dominated by Sr, which is consistent with minimal differentiation involving plagioclase fractionation (fig. 10). REE 

data for the Boulder batholith rocks are also consistent with 

limited magma differentiation. These rocks have minimal 
22 Synthesis of Petrographic, Geochemical, and Isotopic Data for the Boulder Batholith, Southwest Montana

to nonexistent negative Eu anomalies (fig. 11), which also 

reflect insignificant plagioclase fractionation. REE patterns 

for various Boulder batholith rocks are essentially parallel, 

which indicates minimal relative REE fractionation. Consequently, REE-enriched accessory phases, such as titanite, 

apatite, and zircon, cannot have been removed in significant 

quantities from the representative magmas. Noteworthy 

middle (MREE) to HREE depletions among the plutonic 

constituents of the Boulder batholith rocks probably reflect 

variable amphibole fractionation because amphibole fractionation causes Dy/Yb to decrease (Davidson and others, 

2007). The Deer Creek granophyre has distinctly depleted 

LREE abundances that may reflect fractionation of LREEenriched titanite or allanite (Sawka and others, 1984) from 

the associated magma. Alternatively, the aplitic character 

of the Deer Creek granophyre indicates that it could have 

interacted with late-stage (exsolved) magmatic fluids that 

destroyed LREE-enriched accessory minerals and flushed 

LREE from the magmatic system, thereby selectively reducing LREE abundances. Less pronounced LREE depletion, 

characteristic of the Hell Canyon monzogranite, may reflect 

similar processes.

N:\Jeff\den12_cmrk00_0094_pp_dubray\report_figures\figure_12.ai

10

1

100

1,000

Rb Ba Th Nb Ta K La Ce Pb Sr P Nd Zr Hf Sm Eu Ti Dy Y Ho Yb

EXPLANATION

04KL057 Burton Park

98KL003 Butte

98KL001a Butte

03KL067 Butte

98KL004 Butte

03KL068 Butte

02HW-28 Butte

02HW-30 Butte

Macdonald Camp Thunderbird

10KL072 Camp Thunderbird

03KL072 Climax Gulch

10KL073 Colorado Gulch

03KL071 Deer Creek

02HW-32 Deer Creek

04KL058 Donald

04KL058 Hell Canyon

03KL066 Homestake

04KL059 Moose Creek

98KL007 Pulpit Rock

98KL002 Rader Creek

02HW-31b Rader Creek

98KL005 Unionville

Sample/Primtitve mantle

Trace element

Figure 12. Primitive mantle-normalized (Sun and McDonough, 1989) trace-element diagrams for Boulder batholith samples. Trace 

elements arranged in order of increasing compatibility to the right.
Petrogenetic Implications of Geochemical and Isotopic Data 23

N:\Jeff\den12_cmrk00_0094_pp_dubray\report_figures\figure_13_a.ai

A

B

C

140 120 100 60

0.02

0.03

0.04

0.05

0.06

0.07

0.08

40 60 80 100 120 140

10KL085

Granodiorite of Little Butte

64

72

80

88

100 μm

140 120 100 60

0.02

0.03

0.04

0.05

0.06

0.07

0.08

40 60 80 100 120 140

10KL087

Granodiorite of Crystal Creek

58

66

74

82

90

100 μm

140 120 100 60

0.02

0.03

0.04

0.05

0.06

0.07

0.08

40 60 80 100 120 140

10KL072

Monzogranite of Camp Thunderbird

206Pb/238U age 206Pb/238U age 206Pb/238U age

66

74

82

90

100 μm

Age = 81.7 ± 1.4 Ma

(MSWD = 1.64)

Age = 78.4 ± 1.3 Ma

(MSWD = 1.26)

Age = 77.6 ± 1.2 Ma

(MSWD = 1.35)

238U/206Pb

207Pb/206Pb

238U/206Pb

207Pb/206Pb

238U/206Pb

207Pb/206Pb

80

80

80

Figure 13. Photographs of representative zircons and Tera-Wasserburg plots of sensitive high resolution ion microprobe (SHRIMP) 

U-Pb dates from each sample dated in this study. In image pairs, cathodoluminescence images are on the left and transmitted-light 

images are on the right. SHRIMP data for each sample are shown as Tera-Wasserburg plots to the right of images. Data shown by 

gray error ellipses and error bars are excluded from weighted average age calculations. MSWD, mean square of weighted deviates.
24 Synthesis of Petrographic, Geochemical, and Isotopic Data for the Boulder Batholith, Southwest Montana

Figure 13. Photographs of representative zircons and Tera-Wasserburg plots of sensitive high resolution ion microprobe (SHRIMP) 

U-Pb dates from each sample dated in this study. In image pairs, cathodoluminescence images are on the left and transmitted-light 

images are on the right. SHRIMP data for each sample are shown as Tera-Wasserburg plots to the right of images. Data shown 

by gray error ellipses and error bars are excluded from weighted average age calculations. MSWD, mean square of weighted 

deviates.—Continued

N:\Jeff\den12_cmrk00_0094_pp_dubray\report_figures\figure_13_b.ai

E

F

0.02

0.03

0.04

0.05

0.06

0.07

0.08

40 60 80 100 120 140

140 120 100 60

03KL068

Butte Granite

(in Butte mining

 district)

80

71

75

79

83

206Pb/238U age

Age = 76.9 ± 0.8 Ma

(MSWD = 1.3)

0.0

0.1

0.2

0.3

0.4

59 60 64 68 72 76 80 84

110 102 94 86 78

03KL071

Granophyre of Deer Creek

high U (2000-6600 ppm)

high common Pb (3-50% 206Pb)

140 120 100 60

04KL057

Granodiorite of

Burton Park

100 μm

0.02

0.03

0.04

0.05

0.06

0.07

0.08

40 60 80 100 120 140

238U/206Pb

207Pb/206Pb

238U/206Pb

207Pb/206Pb

238U/206Pb

207Pb/206Pb

206Pb/238U age

Age = 77.3 ± 0.8 Ma

(MSWD = 1.5)

80

72

68

84

76

80

D

100 μm

100 μm
Petrogenetic Implications of Geochemical and Isotopic Data 25

N:\Jeff\den12_cmrk00_0094_pp_dubray\report_figures\figure_13_c.ai

140 120 100 60

10KL073

Granodiorite of Colorado Gulch

100 μm

I

0.02

0.03

0.04

0.05

0.06

0.07

0.08

40 60 80 100 120 140

238U/206Pb

207Pb/206Pb

80

62

72

80

88

206Pb/238U age

Age = 76.3 ± 1.2 Ma

(MSWD = 1.47)

140 120 100 60

03KL067

Butte Granite (at Homestake Pass)

100 μm

0.02

0.03

0.04

0.05

0.06

0.07

0.08

40 60 80 100 120 140

238U/206Pb

207Pb/206Pb

H

80

70

74

78

82

206Pb/238U age

Age = 76.5 ± 0.7 Ma

(MSWD = 0.96)

100 μm

238U/206Pb

207Pb/206Pb

0.02

0.03

0.04

0.05

0.06

0.07

0.08

40 60 80 100 120 140

140 120 100 60

03KL072

Granodiorite of

Climax Gulch

80

G

73

77

81

Age = 76.7 ± 0.7 Ma

(MSWD = 1.7)

206Pb/238U age

Figure 13. Photographs of representative zircons and Tera-Wasserburg plots of sensitive high resolution ion microprobe (SHRIMP) 

U-Pb dates from each sample dated in this study. In image pairs, cathodoluminescence images are on the left and transmitted-light 

images are on the right. SHRIMP data for each sample are shown as Tera-Wasserburg plots to the right of images. Data shown 

by gray error ellipses and error bars are excluded from weighted average age calculations. MSWD, mean square of weighted 

deviates.—Continued
26 Synthesis of Petrographic, Geochemical, and Isotopic Data for the Boulder Batholith, Southwest Montana

Figure 13. Photographs of representative zircons and Tera-Wasserburg plots of sensitive high resolution ion microprobe (SHRIMP) 

U-Pb dates from each sample dated in this study. In image pairs, cathodoluminescence images are on the left and transmitted-light 

images are on the right. SHRIMP data for each sample are shown as Tera-Wasserburg plots to the right of images. Data shown 

by gray error ellipses and error bars are excluded from weighted average age calculations. MSWD, mean square of weighted 

deviates.—Continued

J

N:\Jeff\den12_cmrk00_0094_pp_dubray\report_figures\figure_13_d.ai

140 120 100 60

04KL059

Monzogranite of

Moose Creek 50 μm

0.02

0.03

0.04

0.05

0.06

0.07

0.08

40 60 80 100 120 140

238U/206Pb

207Pb/206Pb

80

69

73

77

81

206Pb/238U age

Age = 74.2 ± 0.6 Ma

(MSWD = 1.6)

140 120 100 80 60

03KL069

Granodiorite of Donald 50 μm

0.02

0.03

0.04

0.05

0.06

0.07

0.08

40 60 80 100 120 140

238U/206Pb

207Pb/206Pb

K

72

74

76

78

206Pb/238U age

Age = 74.5 ± 0.5 Ma

(MSWD = 1.7)

140 120 100 60

03KL066

Monzogranite of

Homestake

100 μm

238U/206Pb

207Pb/206Pb

0.02

0.03

0.04

0.05

0.06

0.07

0.08

40 60 80 100 120 140

80

70

74

78

82

Age = 75.8 ± 0.6 Ma

(MSWD = 1.4)

206Pb/238U age

L
Petrogenetic Implications of Geochemical and Isotopic Data 27

Table 4. SHRIMP U-Th-Pb data for zircon from plutons of Boulder batholith, southwest Montana.—Continued

[%, percent; ---, no data]

Sample

Measured 204Pb

206Pb

Measured 207Pb

206Pb

%

common

206Pb

U

(ppm) Th/U 206Pb1

238U (Ma)

err 2

(Ma)

238U3

206Pb

err2

(%)

207Pb3

206Pb

err2

(%) 

Sample 10KL072 (Monzogranite of Camp Thunderbird) [latitude 46.5819o N, longitude –112.27227o W]

10KL072–1.1 0.002432 0.0494 0.21 172 1.33 83.8 2.2 76.23 2.6 .0494 5.4

10KL072–2.1 --- 0.0477 0.01 171 0.89 82.2 2.1 77.95 2.5 .0477 6.2

10KL072–3.1 --- 0.0538 0.76 67 1.03 85.4 2.7 74.45 3.1 .0538 7.7

10KL072–4.1 0.000482 0.0449 –0.35 176 1.03 86.4 2.2 74.34 2.5 .0449 5.2

10KL072–5.1 0.001710 0.0607 1.64 79 1.06 81.8 2.5 77.00 3.0 .0607 6.8

10KL072–6.1 0.000884 0.0511 0.45 124 0.57 78.6 2.1 81.19 2.7 .0511 6.2

10KL072–7.1 0.001596 0.0511 0.44 147 0.64 81.3 2.1 78.49 2.6 .0511 5.7

10KL072–8.1 –0.001121 0.0457 –0.25 135 0.87 85.7 2.4 74.94 2.8 .0457 6.3

10KL072–9.1 0.002733 0.0621 1.83 82 0.84 78.9 2.4 79.70 3.0 .0621 10.8

10KL072–10.1 0.000618 0.0502 0.33 390 1.69 80.1 1.9 79.68 2.3 .0502 3.6

10KL072–11.1 0.001134 0.0524 0.61 170 0.85 78.4 2.0 81.20 2.6 .0524 6.1

10KL072–12.1 0.001211 0.0525 0.61 129 0.59 82.9 2.2 76.81 2.6 .0525 5.7

10KL072–13.1 0.000690 0.0517 0.52 96 0.60 78.4 2.3 81.33 2.9 .0517 7.1

10KL072–14.1 –0.000107 0.0482 0.08 280 1.02 74.9 1.8 85.52 2.4 .0482 4.4

10KL072–15.1 --- 0.0539 0.81 83 0.69 73.9 2.3 86.05 3.1 .0539 7.9

Sample 98BL002 (Rader Creek Granodiorite) [latitude 45.8389o N, longitude –112.3139o W]

98BL002–1.1 0.002872 0.0589 1.43 70 0.96 80.2 2.0 78.79 2.3 .0589 9.6

98BL002–2.1 –0.000331 0.0599 1.55 87 0.95 81.8 1.8 77.06 2.2 .0599 7.2

98BL002–3.1 0.002523 0.1276 10.07 124 1.38 83.4 1.7 69.06 1.9 .1276 4.1

98BL002–4.1 0.004681 0.0660 2.30 67 0.96 84.8 2.4 73.82 2.8 .0660 7.8

98BL002–5.1 --- 0.0605 1.63 68 0.95 79.4 2.0 79.41 2.4 .0605 7.7

98BL002–6.1 --- 0.0828 4.43 61 0.94 80.9 2.2 75.66 2.6 .0828 7.2

98BL002–7.1 0.001243 0.0646 2.14 85 1.02 80.9 1.9 77.45 2.2 .0646 7.5

98BL002–8.1 0.005765 0.0937 5.81 54 1.04 77.3 2.2 78.04 2.7 .0937 7.1

98BL002–9.1 0.003437 0.0920 5.61 174 0.91 74.1 1.3 81.63 1.7 .0920 4.3

98BL002–10.1 0.020579 0.2888 30.50 116 5.36 57.8 1.7 77.23 1.9 .2888 4.0

98BL002–11.1 –0.000517 0.0539 0.78 48 0.98 84.2 2.4 75.47 2.8 .0539 10.2

98BL002–12.1 0.019342 0.3615 39.56 91 1.19 80.9 2.5 47.86 2.1 .3615 2.7

98BL002–13.1 0.003082 0.0736 3.31 187 0.89 71.1 1.2 87.20 1.6 .0736 4.4

98BL002–14.1 0.003866 0.0862 4.87 50 1.06 77.1 2.3 79.11 2.8 .0862 7.8

98BL002–15.1 0.002364 0.0589 1.42 79 1.02 80.4 1.8 78.59 2.2 .0589 7.2

98BL002–16.1 0.001755 0.2002 17.87 76 1.97 436.0 11.9 11.74 2.6 .2002 2.8

98BL002–17.1 0.003273 0.0542 0.82 96 0.86 81.9 1.7 77.54 2.1 .0542 6.9

98BL002–18.1 0.000832 0.0743 3.37 42 1.18 78.3 2.5 79.07 3.1 .0743 9.6

Sample 10KL087 (Granodiorite of Crystal Creek) [latitude 46.4824o N, longitude –111.8448o W]

10KL087–1.1 0.000534 0.0477 0.01 111 1.10 81.3 3.3 78.76 4.1 .0477 6.7

10KL087–1.2 0.000104 0.0469 –0.08 670 0.94 77.8 1.7 82.47 2.2 .0469 2.8

10KL087–2.1 0.000658 0.0491 0.18 184 0.84 82.6 2.0 77.44 2.5 .0491 4.9

10KL087–3.1 --- 0.0509 0.41 95 0.76 81.3 2.3 78.50 2.8 .0509 6.6

10KL087–4.1 –0.002177 0.0453 –0.29 109 1.18 78.8 2.2 81.50 2.7 .0453 6.9

10KL087–5.1 --- 0.0470 –0.08 147 0.56 79.9 2.0 80.29 2.6 .0470 5.5

10KL087–6.1 --- 0.0439 –0.47 105 0.87 81.5 2.3 78.92 2.8 .0439 6.9

10KL087–7.1 --- 0.0537 0.78 80 0.82 75.8 2.3 83.92 3.0 .0537 7.5

10KL087–8.1 0.000262 0.0468 –0.11 459 1.47 80.6 2.7 79.54 3.3 .0468 3.2

10KL087–9.1 0.000662 0.0468 –0.10 96 0.88 77.0 2.2 83.34 2.9 .0468 7.2

10KL087–10.1 0.001317 0.0503 0.34 96 1.08 78.0 2.2 81.92 2.9 .0503 7.1

10KL087–11.1 0.002110 0.0493 0.23 117 0.83 75.3 2.1 84.95 2.7 .0493 6.4

10KL087–12.1 0.001321 0.0456 –0.24 94 0.75 77.2 3.2 83.19 4.1 .0456 7.2
28 Synthesis of Petrographic, Geochemical, and Isotopic Data for the Boulder Batholith, Southwest Montana

Table 4. SHRIMP U-Th-Pb data for zircon from plutons of Boulder batholith, southwest Montana.—Continued

[%, percent; ---, no data]

Sample

Measured 204Pb

206Pb

Measured 207Pb

206Pb

%

common

206Pb

U

(ppm) Th/U 206Pb1

238U (Ma)

err 2

(Ma)

238U3

206Pb

err2

(%)

207Pb3

206Pb

err2

(%) 

10KL087–13.1 0.000967 0.0505 0.40 81 0.80 65.7 2.2 97.28 3.4 .0505 10.7

10KL087–14.1 0.000404 0.0470 –0.07 154 1.15 76.8 2.0 83.48 2.6 .0470 5.7

10KL087–15.1 --- 0.0471 –0.05 91 0.73 74.4 2.3 86.20 3.1 .0471 7.6

Sample 98BL005 (Unionville Granodiorite) [latitude 46.5125o N, longitude –112.2597o W]

98KL005–1.1 0.001001 0.0568 1.16 221 1.18 77.0 1.2 82.26 1.5 .0568 4.6

98KL005–2.1 0.000249 0.0493 0.21 390 1.47 78.9 1.0 80.98 1.2 .0493 3.7

98KL005–3.1 0.000669 0.0538 0.79 242 0.94 79.3 1.2 80.20 1.4 .0538 4.4

98KL005–4.1 0.001281 0.0481 0.07 313 1.10 78.1 1.1 82.01 1.3 .0481 5.2

98KL005–5.1 --- 0.0491 0.20 243 1.18 77.1 1.2 82.95 1.4 .0491 6.6

98KL005–6.1 0.000701 0.0494 0.23 275 1.09 79.3 1.1 80.62 1.4 .0494 4.2

98KL005–7.1 0.001531 0.0570 1.19 198 1.09 78.3 1.3 80.81 1.6 .0570 4.8

98KL005–8.1 0.001224 0.0526 0.64 225 1.15 78.2 1.2 81.42 1.5 .0526 4.7

98KL005–9.1 0.000380 0.0478 0.02 237 1.13 79.4 1.2 80.64 1.4 .0478 4.7

98KL005–10.1 0.000501 0.0518 0.53 323 1.27 78.1 1.0 81.63 1.3 .0518 3.9

98KL005–11.1 0.000340 0.0533 0.72 345 1.42 78.2 1.0 81.36 1.3 .0533 3.8

98KL005–12.1 0.000008 0.0486 0.13 365 0.99 77.9 1.0 82.10 1.3 .0486 3.8

98KL005–13.1 –0.000204 0.0501 0.31 361 1.39 78.8 1.0 81.02 1.2 .0501 3.8

98KL005–14.1 –0.001175 0.0456 –0.24 224 1.23 78.3 1.2 81.99 1.5 .0456 5.1

98KL005–15.1 –0.000831 0.0523 0.60 208 1.19 76.8 1.2 82.94 1.5 .0523 4.9

98KL005–16.1 –0.000366 0.0470 –0.07 232 1.22 77.2 1.2 83.02 1.5 .0470 5.0

Sample 10KL085 (Granodiorite of Little Butte) [latitude –111.8558o N, longitude 46.5195o W]

10KL085–1.1 0.001016 0.0431 –0.58 95 0.77 82.7 2.3 77.93 2.8 .0431 7.3

10KL085–2.1 --- 0.0547 0.89 122 0.83 78.7 2.2 80.68 2.8 .0547 5.7

10KL085–3.1 --- 0.0559 1.06 109 0.95 76.9 2.1 82.43 2.7 .0559 6.1

10KL085–4.1 0.000647 0.0495 0.24 132 1.04 78.8 2.1 81.14 2.6 .0495 5.7

10KL085–5.1 --- 0.0505 0.38 220 1.30 76.5 2.6 83.40 3.3 .0505 4.7

10KL085–6.1 0.000523 0.0506 0.37 192 0.78 81.1 2.0 78.66 2.4 .0506 6.7

10KL085–7.1 0.001198 0.0558 1.04 104 0.63 77.7 2.2 81.63 2.8 .0558 6.2

10KL085–8.1 –0.001141 0.0473 –0.04 162 0.79 77.4 2.0 82.87 2.6 .0473 5.6

10KL085–9.1 0.000810 0.0433 –0.55 108 1.01 79.0 2.2 81.52 2.7 .0433 6.8

10KL085–10.1 0.001429 0.0457 –0.24 129 0.75 79.0 2.1 81.26 2.6 .0457 6.1

10KL085–11.1 --- 0.0509 0.42 144 1.01 79.4 2.0 80.31 2.6 .0509 5.4

10KL085–12.1 0.001820 0.0463 –0.15 86 0.82 75.0 2.2 85.57 2.9 .0463 7.7

10KL085–13.1 0.000816 0.0455 –0.26 144 1.07 77.4 2.0 82.96 2.6 .0455 5.9

10KL085–14.1 --- 0.0500 0.31 127 0.77 74.2 2.0 86.13 2.7 .0500 6.1

10KL085–15.1 --- 0.0477 0.03 103 0.96 74.7 2.1 85.80 2.8 .0477 7.3

10KL085–16.1 0.001039 0.0445 –0.38 203 1.19 74.0 1.8 86.97 2.5 .0445 5.2

10KL085–17.1 --- 0.0456 –0.26 124 0.66 79.6 2.3 80.66 2.8 .0456 6.3

10KL085–18.1 0.000470 0.0523 0.61 142 1.02 73.9 3.0 86.24 4.0 .0523 5.7

10KL085–19.1 --- 0.0506 0.39 101 0.78 71.5 2.1 89.30 2.8 .0506 6.9

Sample 04KL057 (Granodiorite of Burton Park) [latitude 45.8193o N, longitude 112.5203o W]

04KL057–1.1 –0.000293 0.0515 0.50 111 0.75 74.4 1.4 85.25 1.9 0.0558 7.8

04KL057–2.1 0.000000 0.0559 1.06 154 0.72 76.0 1.3 83.41 1.6 0.0559 5.7

04KL057–3.1 0.003059 0.0541 0.82 119 0.69 78.6 1.5 85.74 2.8

04KL057–4.1 0.001454 0.0840 4.43 86 0.46 135.2 2.7 46.34 2.0 0.0630 18.2

04KL057–5.1 0.001368 0.0487 0.14 120 0.80 79.4 1.4 82.68 2.1 0.0280 33.4

04KL057–6.1 –0.001209 0.0503 0.34 117 0.65 78.0 1.5 80.08 2.5 0.0676 19.1

04KL057–7.1 0.001582 0.0509 0.42 136 0.75 76.9 1.4 85.52 2.5 0.0270 56.4

04KL057–8.1 –0.000366 0.0507 0.40 103 0.71 78.3 1.6 81.00 2.0 0.0561 8.4
Petrogenetic Implications of Geochemical and Isotopic Data 29

Table 4. SHRIMP U-Th-Pb data for zircon from plutons of Boulder batholith, southwest Montana.—Continued

[%, percent; ---, no data]

Sample

Measured 204Pb

206Pb

Measured 207Pb

206Pb

%

common

206Pb

U

(ppm) Th/U 206Pb1

238U (Ma)

err 2

(Ma)

238U3

206Pb

err2

(%)

207Pb3

206Pb

err2

(%) 

04KL057–9.1 0.000224 0.0585 1.38 133 0.77 76.3 1.4 83.16 2.2 0.0552 20.3

04KL057–10.1 0.000076 0.1047 3.09 231 0.19 1162.0 7.4 4.91 0.7 0.1036 0.9

04KL057–11.1 0.000000 0.0532 0.72 171 0.62 71.8 1.2 88.68 1.6 0.0532 5.7

04KL057–12.1 –0.000370 0.0489 0.16 117 0.59 80.4 1.5 79.02 1.9 0.0543 8.8

04KL057–13.1 0.001559 0.0525 0.63 185 1.26 77.0 1.1 85.18 1.9 0.0290 35.8

04KL057–14.1 0.000945 0.0516 0.51 117 0.91 79.3 1.5 81.77 2.0 0.0375 19.6

04KL057–15.1 0.001686 0.0574 1.25 154 0.70 74.2 1.2 88.00 2.1 0.0321 35.2

04KL057–16.1 0.002399 0.0504 0.36 135 0.69 78.3 1.4 85.34 2.4

04KL057–17.1 0.001311 0.0560 1.06 87 0.77 77.1 1.7 84.33 2.4 0.0364 27.5

04KL057–18.1 0.000958 0.0500 0.31 89 0.80 75.8 1.6 85.84 2.7 0.0357 41.0

04KL057–19.1 0.001902 0.0462 –0.18 111 0.68 79.6 1.5 83.60 2.5 0.0171 84.1

04KL057–20.1 –0.002302 0.0494 0.24 89 0.73 76.0 1.6 80.67 3.1 0.0819 21.4

04KL057–21.1 –0.002405 0.0423 –0.67 118 0.69 78.0 1.4 79.20 2.6 0.0764 19.7

Sample 03KL071 (Granophyre of Deer Creek) [latitude 45.8595o N, longitude 112.4647o W]

03KL071–1.1 0.016349 0.2902 35.21 2365 0.25 67.1 6.4 88.94 4.8

03KL071–2.1 0.009091 0.1809 19.35 2650 0.46 68.6 2.4 90.75 1.5 0.0677 34.1

03KL071–3.1 0.005501 0.1260 12.31 2614 0.23 76.3 1.7 81.99 1.0 0.0624 20.9

03KL071–4.1 0.011057 0.2129 23.99 4284 0.34 68.9 3.2 89.00 1.7 0.0772 37.6

03KL071–5.1 0.001829 0.0771 4.64 3137 0.26 76.2 0.7 83.00 0.5 0.0557 7.7

03KL071–5.2 0.004005 0.1078 9.45 2046 0.12 75.4 1.9 83.11 1.8 0.0613 29.3

03KL071–6.1 0.006221 0.1417 11.88 3873 0.27 74.2 1.3 86.08 1.0 0.0503 31.2

03KL071–7.1 0.007027 0.1485 15.84 4071 0.15 75.9 2.4 81.68 1.8 0.0679 28.2

03KL071–8.1 0.027167 0.4411 49.69 6633 0.48 61.9 7.8 105.42 6.0

03KL071–9.1 0.003796 0.0986 8.02 5095 0.23 77.0 1.3 82.35 1.2 0.0541 22.9

Sample 03KL068 (Butte Granite, in Butte mining district) [latitude 46.0390o N, longitude 112.5320o W]

03KL068–1.1 –0.000286 0.0511 0.45 390 1.16 74.5 0.9 85.20 1.2 0.0552 5.4

03KL068–2.1 0.000000 0.0525 0.63 160 0.86 75.5 1.2 84.37 1.5 0.0525 5.0

03KL068–3.1 –0.000748 0.0512 0.46 257 0.69 75.5 1.6 83.30 2.3 0.0620 9.6

03KL068–4.1 0.001205 0.0480 0.06 178 0.74 77.2 1.2 84.90 1.8 0.0298 27.0

03KL068–5.1 0.001139 0.0498 0.29 186 1.03 75.2 1.1 86.79 1.6 0.0327 18.5

03KL068–6.1 0.007084 0.1611 14.32 264 0.67 76.7 2.2 82.41 3.0 0.0580 43.3

03KL068–7.1 –0.000863 0.0490 0.18 132 0.69 76.4 1.3 82.38 2.0 0.0615 14.6

03KL068–8.1 0.000862 0.0494 0.23 161 0.70 77.2 1.2 84.11 1.7 0.0365 18.0

03KL068–9.1 0.000768 0.0453 –0.28 147 0.65 76.9 1.3 84.75 1.7 0.0338 16.0

03KL068–10.1 0.000042 0.0510 0.43 175 0.82 76.3 1.2 83.72 1.6 0.0504 9.9

03KL068–11.1 –0.001060 0.0513 0.47 149 0.71 75.8 1.2 82.48 2.0 0.0665 14.2

03KL068–12.1 0.000641 0.0448 –0.35 159 0.72 79.0 1.3 82.36 1.8 0.0352 20.2

03KL068–13.1 0.001657 0.0513 0.47 95 0.86 78.5 1.5 83.84 2.7 0.0262 62.2

03KL068–14.1 0.000746 0.0479 0.05 135 0.72 77.5 1.3 83.83 1.7 0.0368 13.1

03KL068–15.1 0.000548 0.0497 0.26 142 0.72 79.0 1.3 81.72 1.7 0.0415 12.2

Sample 03KL072 (Granodiorite of Climax Gulch) [latitude 45.8702o N, longitude 112.5519o W]

03KL072–1.1 0.000297 0.0497 0.28 315 0.62 74.5 0.9 86.25 1.3 0.0453 5.9

03KL072–2.1 0.000509 0.0482 0.08 382 0.57 77.0 0.9 83.97 1.3 0.0406 9.3

03KL072–3.1 –0.000677 0.0492 0.20 292 0.75 77.9 1.3 81.11 1.8 0.0590 9.7

03KL072–4.1 0.000295 0.0507 0.40 333 0.60 75.3 1.0 85.27 1.3 0.0463 6.6

03KL072–5.1 0.002076 0.0532 0.71 187 0.72 76.6 1.5 86.39 2.6 0.0216 65.6

03KL072–6.1 0.000102 0.0508 0.41 301 0.64 77.9 1.0 82.07 1.4 0.0494 8.8

03KL072–7.1 –0.000257 0.0474 –0.02 403 0.65 75.7 0.9 84.25 1.2 0.0512 5.1

03KL072–8.1 –0.000143 0.0472 –0.05 260 0.65 76.5 1.0 83.63 1.3 0.0493 5.1
30 Synthesis of Petrographic, Geochemical, and Isotopic Data for the Boulder Batholith, Southwest Montana

Table 4. SHRIMP U-Th-Pb data for zircon from plutons of Boulder batholith, southwest Montana.—Continued

[%, percent; ---, no data]

Sample

Measured 204Pb

206Pb

Measured 207Pb

206Pb

%

common

206Pb

U

(ppm) Th/U 206Pb1

238U (Ma)

err 2

(Ma)

238U3

206Pb

err2

(%)

207Pb3

206Pb

err2

(%) 

03KL072–9.1 0.000551 0.0477 0.03 453 0.52 75.7 0.9 85.46 1.2 0.0395 4.9

03KL072–10.1 0.000031 0.0478 0.04 688 0.56 76.8 0.8 83.44 1.1 0.0474 4.6

03KL072–11.1 0.001052 0.0518 0.54 208 0.61 76.4 1.1 85.10 1.7 0.0361 20.5

03KL072–12.1 0.000000 0.0479 0.04 605 0.54 78.6 0.9 81.53 1.1 0.0479 3.0

03KL072–13.1 0.000344 0.0498 0.28 527 0.59 75.9 0.9 84.71 1.3 0.0447 5.2

03KL072–14.1 0.000195 0.0527 0.65 331 0.92 77.3 1.0 82.70 1.2 0.0499 4.9

03KL072–15.1 –0.000105 0.0497 0.28 518 0.62 74.4 0.8 85.75 1.1 0.0513 3.6

Sample 03KL067 (Butte Granite, at Homestake Pass) [latitude 45.9230o N, longitude 112.4170o W]

03KL067–1.1 0.000000 0.0510 0.44 268 0.64 74.3 1.3 85.83 1.7 0.0510 4.0

03KL067–2.1 0.000000 0.0483 0.09 392 0.62 76.6 1.0 83.57 1.3 0.0483 3.6

03KL067–3.1 0.002992 0.0577 1.28 66 0.55 76.6 1.8 87.45 3.1

03KL067–4.1 0.000179 0.0500 0.30 778 0.69 77.3 0.8 82.91 1.0 0.0473 2.9

03KL067–5.1 0.000000 0.0517 0.53 271 0.65 76.1 1.0 83.73 1.3 0.0517 4.1

03KL067–6.1 –0.000281 0.0474 –0.03 191 0.66 78.0 1.2 81.69 1.5 0.0515 7.7

03KL067–7.1 0.000349 0.0506 0.38 350 0.54 76.3 0.9 84.20 1.2 0.0454 5.7

03KL067–8.1 –0.000230 0.0481 0.07 509 0.60 75.7 1.0 84.27 1.4 0.0515 5.3

03KL067–9.1 –0.000820 0.0510 0.45 174 0.70 74.6 1.1 84.22 1.7 0.0629 12.2

03KL067–11.1 –0.000046 0.0442 –0.41 298 0.62 75.8 1.0 84.78 1.2 0.0449 3.9

03KL067–12.1 –0.000202 0.0494 0.23 234 0.63 76.9 1.1 82.87 1.4 0.0523 5.7

03KL067–13.1 0.000390 0.0480 0.07 395 0.55 76.0 1.0 84.91 1.4 0.0423 6.2

03KL067–14.1 0.000000 0.0478 0.03 249 0.65 75.1 1.0 85.26 1.3 0.0478 4.3

03KL067–15.1 0.000000 0.0499 0.30 264 0.65 77.8 1.0 82.08 1.3 0.0499 4.0

03KL067–16.1 0.000277 0.0475 –0.01 356 0.63 77.0 0.9 83.70 1.2 0.0434 6.6

Sample 98BL007 (Monzogranite of Pulpit Rock) [latitude 46.1960o N, longitude –112.1330o W]

98BL007–1.1 0.000335 0.0515 0.50 466 0.60 75.8 0.9 84.10 1.2 .0515 3.4

98BL007–2.1 –0.001166 0.0573 1.23 215 0.60 75.3 1.2 84.05 1.6 .0573 4.8

98BL007–3.1 0.000806 0.0491 0.20 329 0.58 76.9 1.0 83.13 1.3 .0491 4.1

98BL007–4.1 –0.000194 0.0514 0.48 325 0.57 76.4 1.0 83.50 1.3 .0514 4.1

98BL007–5.1 0.001000 0.0543 0.85 507 0.59 77.9 0.9 81.55 1.1 .0543 3.8

98BL007–6.1 0.000967 0.0552 0.96 399 0.61 76.3 0.9 83.20 1.2 .0552 3.5

98BL007–7.1 0.000513 0.0521 0.58 504 0.53 77.0 0.9 82.74 1.2 .0521 3.2

98BL007–8.1 0.000718 0.0561 1.09 406 0.55 75.4 0.9 84.05 1.2 .0561 3.4

98BL007–9.1 0.001088 0.0499 0.30 247 0.54 73.7 1.1 86.70 1.5 .0499 4.7

98BL007–10.1 0.000795 0.0486 0.14 362 0.59 77.2 1.0 82.91 1.3 .0486 3.9

98BL007–11.1 0.000019 0.0508 0.42 271 0.65 75.8 1.1 84.16 1.4 .0508 4.4

98BL007–12.1 0.001229 0.0517 0.53 378 0.65 75.4 1.0 84.50 1.3 .0517 3.7

Sample 10KL073 (Granodiorite of Colorado Gulch) [latitude 46.5761o N, longitude –112.2009o W]

10KL073–1.1 0.000242 0.0492 0.21 610 0.26 77.2 1.7 82.82 2.2 .0492 3.0

10KL073–2.1 0.000213 0.0492 0.20 305 0.62 77.7 1.9 82.26 2.4 .0492 4.0

10KL073–3.1 --- 0.0451 –0.31 221 0.53 76.7 1.9 83.77 2.4 .0451 6.3

10KL073–4.1 0.000659 0.0509 0.42 200 0.54 77.2 2.0 82.69 2.6 .0509 4.9

10KL073–5.1 --- 0.0466 –0.13 234 0.45 77.6 1.9 82.67 2.4 .0466 6.3

10KL073–6.1 –0.000514 0.0466 –0.13 515 0.51 79.3 1.8 80.85 2.2 .0466 3.2

10KL073–7.1 –0.000450 0.0478 0.01 205 0.46 83.9 2.0 76.29 2.4 .0478 4.7

10KL073–8.1 0.000324 0.0499 0.30 265 0.71 77.0 1.8 82.96 2.4 .0499 4.3

10KL073–9.1 –0.000662 0.0489 0.18 330 0.53 74.1 1.7 86.34 2.3 .0489 4.0

10KL073–10.1 0.000105 0.0482 0.08 597 0.63 77.4 1.7 82.67 2.1 .0482 2.7

10KL073–11.1 0.000642 0.0497 0.28 213 0.58 73.7 1.9 86.74 2.5 .0497 5.4

10KL073–12.1 –0.001032 0.0458 –0.21 288 0.37 72.2 1.7 89.03 2.4 .0458 4.6
Petrogenetic Implications of Geochemical and Isotopic Data 31

Table 4. SHRIMP U-Th-Pb data for zircon from plutons of Boulder batholith, southwest Montana.—Continued

[%, percent; ---, no data]

Sample

Measured 204Pb

206Pb

Measured 207Pb

206Pb

%

common

206Pb

U

(ppm) Th/U 206Pb1

238U (Ma)

err 2

(Ma)

238U3

206Pb

err2

(%)

207Pb3

206Pb

err2

(%) 

10KL073–13.1 0.002064 0.0490 0.20 146 0.89 69.3 1.9 92.29 2.7 .0490 6.0

10KL073–14.1 --- 0.0469 –0.07 734 1.30 70.5 1.5 91.03 2.2 .0469 2.8

10KL073–15.1 0.000177 0.0460 –0.18 507 0.56 71.5 1.6 89.86 2.2 .0460 3.2

Sample 03KL066 (Monzogranite of Homestake) [latitude 45.9211o N, longitude 112.4209o W]

03KL066–1.1 0.000790 0.0540 0.82 329 0.63 74.8 0.9 86.20 1.2 0.0423 7.6

03KL066–2.1 0.000500 0.0471 –0.06 248 0.62 75.8 1.0 85.34 1.4 0.0396 10.9

03KL066–3.1 0.001199 0.0484 0.12 189 0.60 73.8 1.2 88.74 1.8 0.0304 29.5

03KL066–4.1 0.002238 0.0822 4.37 282 0.63 77.8 1.4 82.20 2.1 0.0493 23.0

03KL066–5.1 0.000000 0.0494 0.23 467 0.49 76.0 0.9 84.10 1.1 0.0494 3.1

03KL066–6.1 0.001335 0.0479 0.05 196 0.68 76.6 1.1 85.71 1.5 0.0278 20.3

03KL066–7.1 0.001071 0.0526 0.65 217 0.64 75.1 1.1 86.53 1.7 0.0366 22.9

03KL066–8.1 0.000000 0.0470 –0.06 371 0.62 76.3 1.3 84.05 1.7 0.0470 3.6

03KL066–10.1 0.000588 0.0512 0.47 270 0.55 74.6 1.0 86.43 1.5 0.0425 14.8

03KL066–11.1 –0.000109 0.0494 0.25 1106 0.50 74.5 0.7 85.69 1.0 0.0510 3.1

03KL066–12.1 0.000679 0.0454 –0.26 216 0.68 75.6 1.2 86.12 1.8 0.0353 17.9

03KL066–13.1 0.000586 0.0508 0.42 167 0.68 74.9 1.2 86.12 1.7 0.0421 13.1

03KL066–14.1 0.000456 0.0500 0.30 393 0.65 78.3 1.0 82.33 1.2 0.0432 6.5

03KL066–15.1 0.000364 0.0495 0.25 765 0.62 76.2 0.8 84.47 1.0 0.0441 4.5

03KL066–16.1 0.000206 0.0484 0.11 558 0.56 75.2 0.9 85.45 1.2 0.0454 4.4

03KL066–17.1 0.000470 0.0466 –0.11 215 0.62 74.1 1.1 87.35 1.5 0.0396 11.4

03KL066–18.1 0.000547 0.0501 0.33 364 0.53 74.7 0.9 86.44 1.4 0.0420 12.5

Sample 98BL004 (Butte Granite, at Bison Creek) [latitude 46.1917o N, longitude –112.3444o W]

98BL004–1.1 0.002693 0.0547 0.91 111 0.72 73.3 1.5 86.63 1.9 .0547 6.3

98BL004–2.1 0.001813 0.0529 0.68 149 0.64 74.9 1.3 85.00 1.7 .0529 5.7

98BL004–3.1 0.002741 0.0541 0.83 150 0.80 77.0 1.3 82.55 1.7 .0541 5.7

98BL004–4.1 0.001683 0.0524 0.61 188 0.71 76.6 1.2 83.11 1.6 .0524 5.0

98BL004–5.1 0.006364 0.1173 8.80 293 1.24 73.9 1.4 79.09 1.5 .1173 6.3

98BL004–6.1 0.000117 0.0523 0.60 191 0.99 78.3 1.4 81.37 1.7 .0523 5.6

98BL004–7.1 0.000084 0.0502 0.34 688 1.48 77.5 0.9 82.39 1.1 .0502 5.2

98BL004–8.1 –0.000461 0.0564 1.12 200 1.04 73.3 1.3 86.50 1.7 .0564 5.5

98BL004–9.1 0.000471 0.0530 0.69 413 1.15 74.5 1.0 85.47 1.3 .0530 3.9

98BL004–10.1 –0.000968 0.0566 1.15 149 0.89 74.9 1.5 84.55 2.0 .0566 6.6

98BL004–11.1 0.000583 0.0554 1.00 319 1.26 77.1 1.1 82.32 1.4 .0554 4.2

98BL004–12.1 0.000101 0.0545 0.88 266 0.60 75.8 1.2 83.83 1.5 .0545 4.9

98BL004–13.1 0.001029 0.0602 1.59 119 0.78 76.2 1.7 82.71 2.2 .0602 7.0

98BL004–14.1 0.002641 0.0516 0.52 157 0.78 75.8 1.5 84.07 1.9 .0516 6.3

98BL004–15.1 0.001932 0.0615 1.77 156 0.72 72.2 1.4 87.17 1.9 .0615 7.9

98BL004–16.1 --- 0.0877 4.14 178 0.64 383.2 4.5 15.65 1.2 .0877 2.0

98BL004–17.1 0.000744 0.0505 0.37 393 0.64 76.5 1.0 83.41 1.3 .0505 4.1

98BL004–18.1 –0.000327 0.0541 0.84 730 0.41 74.5 0.8 85.35 1.1 .0541 3.0

98BL004–19.1 0.003658 0.0822 4.38 181 0.74 73.0 1.4 83.93 1.8 .0822 4.8

98BL004–20.1 –0.000312 0.0531 0.72 284 0.68 72.9 1.1 87.27 1.5 .0531 4.7

Sample 03KL069 (Granodiorite of Donald) [latitude 45.8493o N, longitude 112.4277o W]

03KL069–1 0.000565 0.0503 0.35 343 0.85 73.6 0.7 87.74 1.0 0.0419 10.2

03KL069–2 0.000192 0.0497 0.27 559 1.41 74.7 0.7 85.84 0.9 0.0468 3.5

03KL069–3 0.000000 0.0463 –0.15 424 1.12 75.6 0.6 84.85 0.8 0.0463 2.9

03KL069–4 0.000209 0.0478 0.04 864 1.29 74.9 0.5 85.91 0.7 0.0447 3.5

03KL069–5 –0.000411 0.0471 –0.05 439 1.24 73.0 0.6 87.23 0.9 0.0531 5.8

03KL069–6 0.000462 0.0514 0.49 255 0.66 74.0 0.8 86.96 1.1 0.0445 7.7
32 Synthesis of Petrographic, Geochemical, and Isotopic Data for the Boulder Batholith, Southwest Montana

Table 4. SHRIMP U-Th-Pb data for zircon from plutons of Boulder batholith, southwest Montana.—Continued

[%, percent; ---, no data]

Sample

Measured 204Pb

206Pb

Measured 207Pb

206Pb

%

common

206Pb

U

(ppm) Th/U 206Pb1

238U (Ma)

err 2

(Ma)

238U3

206Pb

err2

(%)

207Pb3

206Pb

err2

(%) 

03KL069–7.1 –0.000104 0.0462 –0.16 412 0.74 73.5 0.8 87.21 1.1 0.0477 3.5

03KL069–8.1 0.000813 0.0517 0.54 217 0.70 73.8 0.8 87.73 1.1 0.0396 6.6

03KL069–9.1 –0.000347 0.0464 –0.14 447 1.57 74.6 0.6 85.44 0.9 0.0515 5.7

03KL069–10.1 –0.000174 0.0494 0.24 1167 1.10 75.1 0.5 84.87 0.6 0.0520 3.1

03KL069–11.1 –0.000077 0.0497 0.27 223 0.70 74.1 0.8 86.11 1.0 0.0508 4.2

03KL069–12.1 0.000221 0.0466 –0.11 275 0.68 73.7 0.7 87.37 1.0 0.0434 7.3

03KL069–13.1 –0.000355 0.0488 0.16 245 1.44 74.9 0.8 84.84 1.0 0.0540 3.7

03KL069–14.1 –0.000353 0.0478 0.04 530 1.19 75.7 0.6 84.13 0.8 0.0530 5.1

03KL069–15.1 –0.000124 0.0477 0.03 470 1.06 75.7 0.9 84.44 1.2 0.0496 4.1

03KL069–15.2 0.000003 0.1873 3.11 997 0.51 2452.7 15.8 2.09 0.5 0.1873 0.2

Sample 04KL059 (Monzogranite of Moose Creek) [latitude 45.7394o N, longitude 112.6740o W]

04KL059–1.1 0.000420 0.0485 0.14 827 1.35 73.0 0.6 88.35 0.8 0.0423 7.5

04KL059–2.1 0.000558 0.0488 0.16 247 0.93 73.8 1.0 87.58 1.5 0.0405 11.3

04KL059–3.1 0.000827 0.0478 0.03 345 1.95 75.9 0.9 85.67 1.4 0.0354 16.4

04KL059–4.1 0.000702 0.0540 0.83 343 0.59 72.5 0.9 88.88 1.3 0.0436 11.3

04KL059–5.1 –0.000068 0.0495 0.25 1152 1.03 74.4 0.5 85.80 0.7 0.0505 2.7

04KL059–6.1 –0.000044 0.0508 0.42 597 1.35 74.2 0.7 85.91 0.9 0.0515 3.5

04KL059–7.1 –0.000145 0.0471 –0.06 739 1.01 77.1 0.6 82.95 0.8 0.0492 3.7

04KL059–8.1 0.000989 0.0589 1.43 2662 0.15 79.2 0.3 81.23 0.5 0.0443 6.5

04KL059–9.1 0.000396 0.0527 0.65 381 1.09 75.9 0.8 84.51 1.3 0.0468 12.4

04KL059–10.1 0.000255 0.0506 0.40 382 1.85 73.6 0.8 87.21 1.2 0.0469 8.4

04KL059–11.1 0.000358 0.0498 0.29 545 2.20 74.4 0.7 86.46 1.0 0.0445 8.1

04KL059–12.1 0.000590 0.0484 0.09 579 0.95 80.7 0.7 80.15 0.9 0.0396 8.6

04KL059–13.1 0.000057 0.0472 –0.04 527 0.66 73.9 0.7 86.80 1.0 0.0464 5.8

04KL059–14.1 0.000055 0.0469 –0.08 182 0.62 74.6 1.2 86.01 1.7 0.0461 12.9

04KL059–15.1 –0.000370 0.0515 0.51 329 1.05 74.4 0.9 85.09 1.2 0.0569 5.8

Sample 04KL058 (Monzogranite of Hell Canyon) [latitude 45.6353o N, longitude 112.3435o W]

04KL058–1.1 0.000126 0.0505 0.38 440 0.55 73.3 0.8 87.35 1.0 0.0487 5.3

04KL058–2.1 0.000918 0.0482 0.09 346 0.56 73.9 0.8 88.21 1.3 0.0344 17.8

04KL058–3.1 –0.000333 0.0501 0.33 378 0.73 73.6 0.8 86.23 1.1 0.0550 6.1

04KL058–4.1 0.000579 0.0475 0.00 418 0.73 72.3 0.8 89.58 1.2 0.0388 11.5

04KL058–5.1 0.000294 0.0495 0.26 270 0.62 72.8 1.0 88.33 1.3 0.0451 8.2

04KL058–6.1 0.000102 0.0476 0.02 531 0.82 74.4 0.7 86.28 0.9 0.0461 4.7

04KL058–7.1 –0.000036 0.0479 0.05 505 0.67 74.3 0.7 86.17 1.0 0.0484 3.7

04KL058–8.1 –0.000274 0.0506 0.39 305 0.62 74.2 0.9 85.57 1.3 0.0546 6.5

04KL058–9.1 0.000466 0.0494 0.24 291 0.56 72.4 0.9 89.14 1.3 0.0424 10.4

04KL058–10.1 0.000348 0.0519 0.56 505 0.83 73.9 0.7 86.76 1.0 0.0468 7.1

04KL058–11.1 0.000747 0.0477 0.02 274 0.59 74.5 0.9 87.20 1.4 0.0365 14.2

04KL058–12.1 –0.000083 0.0517 0.54 479 0.87 72.6 0.7 87.71 0.9 0.0529 4.1

04KL058–13.1 0.000425 0.0509 0.44 470 0.87 72.9 0.7 88.23 1.1 0.0447 10.6

04KL058–14.1 0.000012 0.0504 0.36 334 0.62 74.1 0.9 86.17 1.4 0.0502 11.3

04KL058–15.1 –0.000081 0.0490 0.19 558 1.09 75.5 0.7 84.62 0.9 0.0502 3.3 1 206Pb/238U ages corrected for common Pb using the 207Pb-correction method. Decay constants from Steiger and Jäger (1977). 2

 1σ errors.

3

 Radiogenic ratios, corrected for common Pb using the 204Pb-correction method, based on the Stacey and Kramers (1975) model.
Petrogenetic Implications of Geochemical and Isotopic Data 33

The weakly adakitic character of the Boulder batholith 

rocks implies an affinity with magmas whose petrogenesis 

included a subducted-slab partial melt component. However, 

the equivocal nature of this association and the finding by 

Richards and Kerrich (2007) that adakitic rocks need not 

involve subducted slab components indicate that the Boulder 

batholith rocks reflect magma genesis by ordinary processes 

typically associated with arc magmatism and subsequent, 

extensive open system petrogenesis. Consequently, petrogenesis of the Boulder batholith probably does not involve a 

subducted slab component. The Boulder batholith rocks are 

not ore metal enriched; consequently, processes that fostered 

formation of mineral deposits associated with these rocks do 

not require involvement of metal-enriched magma. Although 

weakly adakitic, Boulder batholith rocks are known to be 

as much as 10 m.y. older than spatially associated porphyry 

copper-molybdenum deposits (Lund and others, 2002), which 

is in accord with observations by Richards and Kerrich (2007) 

that adakitic compositions do not necessarily identify plutons 

with enhanced potential for spatially and genetically associated porphyry copper deposits.

Table 5. Summary of age data for plutons of Boulder batholith, 

southwest Montana. 

Sample

number Unit Age (Ma)

10KL072 Monzogranite of Camp Thunderbird 81.7 ± 1.4

98BL002 Rader Creek Granodiorite 1

80.7 ± 0.8

10KL087 Granodiorite of Crystal Creek 78.4 ± 1.3

98BL005 Unionville Granodiorite 1

78.2 ± 0.5

10KL085 Granodiorite of Little Butte 77.6 ± 1.2

04KL057 Granodiorite of Burton Park 77.3 ± 0.8

03KL071 Granophyre of Deer Creek 76.9 ± 0.6

03KL068 Butte Granite (in Butte mining district) 76.9 ± 0.8

03KL072 Granodiorite of Climax Gulch 76.7 ± 0.7

03KL067 Butte Granite (at Homestake Pass) 76.5 ± 0.7

98BL007 Monzogranite of Pulpit Rock 1

76.3 ± 0.5

10KL073 Granodiorite of Colorado Gulch 76.3 ± 1.2

03KL066 Monzogranite of Homestake 75.8 ± 0.6

98BL004 Butte Granite (at Bison Creek) 1

74.7 ± 0.6

03KL069 Granodiorite of Donald 74.5 ± 0.5

04KL059 Monzogranite of Moose Creek 74.2 ± 0.6

04KL058 Monzogranite of Hell Canyon 73.7 ± 0.6 1

Average of data from Lund and others (2002) and new data (this study).

Figure 14. Epsilon neodymium versus initial strontium isotopic data for Boulder batholith samples. Fields for specific batholiths and 

plutons in Idaho and Montana from Taylor and others (2007); Unruh and others (2008).

−20

−10

0

10

0.702 0.704 0.708 0.712 0.714

εNdi

0.706 0.710

Western Idaho and

eastern Oregon arc plutons

Salmon River

suture zone

plutons

Big Belt Mountains

Atlanta lobe

Idaho batholith

Pioneer

batholith

Southwest Montana

basement and

Mesozoic intrusions

Butte and Pulpit Rock

Rader Creek, Homestake,

and Donald

Unionville, Burton Park,

Moose Creek, Climax Gulch,

and Hell Canyon

87Sr/89Sri

Boulder batholith sample

EXPLANATION
Table 6. Rb-Sr and Sm-Nd isotopic data for samples of the Boulder batholith, southwest Montana. Sample Pluton Rb Sr Sm Nd Age 87Rb/86Sr 87Sr/86Sr Uncertainty 2σ 147Sm/144Nd 143Nd/144Nd Uncertainty 2σ Sri N

m

98KL002 Rader Creek 67.2 703 4.5 23.9 80.7 0.267 0.70726 0.00001 0.116 0.511647 0.000011 0.70695 0.5

98KL005 Unionville 153 481 5.2 29.6 78.2 0.887 0.70815 0.00002 0.108 0.511868 0.000007 0.70716 0.5

04KL057 Burton Park 134 653 4.6 28.4 77.6 0.573 0.70803 0.00001 0.100 0.511813 0.000009 0.70740 0.5

98KL004 Butte 169 431 4.6 30 74.7 1.094 0.70809 0.00001 0.095 0.511972 0.000007 0.70693 0.5

03KL067 Butte 152 530 5.3 31.7 76.4 0.800 0.70813 0.00001 0.103 0.511981 0.000012 0.70726 0.5

03KL068 Butte 151 467 4.6 29.8 76.5 0.902 0.70813 0.00001 0.095 0.511969 0.000007 0.70715 0.5

98KL007 Pulpit Rock 171 430 3.9 26.2 76.3 1.110 0.70805 0.00001 0.092 0.511975 0.000008 0.70685 0.5

03KL072 Climax Gulch 146 494 3.4 21.5 76.3 0.825 0.70857 0.00001 0.098 0.511750 0.000006 0.70768 0.5

03KL066 Homestake 197 256 4.1 22.2 75.3 2.147 0.70932 0.00001 0.114 0.511626 0.000009 0.70702 0.5

03KL069 Donald 112 561 2.7 20.3 74.7 0.557 0.70758 0.00002 0.082 0.511406 0.000009 0.70699 0.5

04KL059 Moose Creek 113 235 3.7 21.4 74.2 1.342 0.70962 0.00001 0.107 0.511835 0.000008 0.70821 0.5

04KL058 Hell Canyon 107 587 2.2 13.5 73.7 0.509 0.70945 0.00001 0.101 0.511749 0.000011 0.70892 0.5
References Cited 35

Conclusions

The volume of the Late Cretaceous Boulder batholith is 

dominated by the large Butte Granite intrusion but includes at 

least 14 additional intrusive rock units. The petrographic, geochemical, and geochronologic characteristics of each of these 

units are distinct and foster identification of each intrusion. 

Compositions of the Boulder batholith rocks range from moderately mafic granodiorite to monzogranite and felsic granophyre. Standard intrusive rock geochemical metrics for the 

Boulder batholith rocks are consistent with their genesis being 

related to subduction and continental magmatic arc processes. 

A subduction-related origin for these rocks, well inboard from 

the edge of the Late Cretaceous western margin of North 

America, requires a high convergence rate and consequent 

shallow subduction. Accordingly, the critical depth of subduction-related magmatism shifted eastward over time. Subsequently (Late Cretaceous to Paleocene), when the convergence 

rate decreased along the western margin of North America, the 

subducted slab steepened and hinge rollback ensued.

Trace element data for the Boulder batholith rocks indicate that the magmas experienced minimal internal differentiation. Sr dominates large ion lithophile abundances and negative Eu anomalies are of limited magnitude. Consequently, 

magmas represented by these rocks experienced only modest 

plagioclase fractionation. REE patterns are remarkably similar 

between plutons. The lack of relative pattern rotation also 

indicates the limited role of accessory mineral fractionation 

in the genesis of these rocks. Subtle but systematic MREE to 

HREE depletions of the Boulder batholith rocks show that 

amphibole may have significantly affected their petrogenesis. 

Radiogenic isotope data for these rocks are also consistent 

with a subduction-related origin, including a significant mantle 

component. Between-pluton radiogenic isotopic variations 

probably reflect a combination of processes, including variable 

assimilation of crustal components by mantle-derived magmas 

as well as basement isotopic inhomogeneities. Although the 

Boulder batholith rocks have a weak adakitic geochemical 

affinity, compelling evidence for involvement of a subducted 

slab component in the genesis of these rocks is absent.

Magmas represented by the Boulder batholith rocks do 

not preserve evidence of unusual ore-metal enrichment. Consequently, ore-forming processes responsible for genesis of 

mineral deposits do not seem to require a particularly unique 

magma composition. Rather, the genesis of associated mineral 

deposits seems to reflect a combination of factors, including: 

a magma (source) containing the ore metals (though not in 

unusual abundances), hydrothermal fluids with the appropriate ligands and redox conditions (transport medium), conductive structural features (conduits), and reactive/receptive host 

rocks (sink).

Acknowledgments

This work was funded by the USGS Mineral Resources 

Program. We thank Bettina Wiegand for Sr and Nd whole-rock 

isotopic analyses. We also gratefully acknowledge reviews 

by C.N. Mercer and C.D. Taylor that helped considerably 

improve this synthesis.

References Cited

Anders, E., and Ebihara, Mitsuru, 1982, Solar-system abundances of the elements: Geochimica et Cosmochimica Acta, 

v. 46, p. 2362–2380.

Bateman, P.C., 1992, Plutonism in the central part of the Sierra 

Nevada Batholith, California: U.S. Geological Survey Professional Paper 1483, 186 p.

Becraft, G.E., and Pinckney, D.M., 1961, Preliminary geologic 

map of the northwest quarter of the Boulder quadrangle, 

Montana: U.S. Geological Survey Miscellaneous Field 

Studies Map MF–183, scale 1:24,000.

Becraft, G.E., Pinckney, D.M., and Rosenblum, S., 1963, 

Geology and mineral deposits of the Jefferson City quadrangle, Jefferson and Lewis and Clark counties, Montana, U.S. 

Geological Survey Professional Paper 428, 101 p.

Berger, B.R., Hildenbrand, T.G., and O’Neill, J.M., 2011, 

Control of Precambrian basement deformation zones on 

emplacement of the Laramide Boulder batholith and Butte 

mining district, Montana, United States: U.S. Geological 

Survey Scientific Investigations Report 2011–5016, 39 p.

Biehler, S., and Bonini, W.E., 1969, A regional gravity study 

of the Boulder Batholith, Montana: Geological Society of 

America Memoir 115, p. 401–422.

Black, L.P., Kamo, S.L., Allen, C.M., Davis, D.W., Aleinikoff, J.N., Valley, J.W., Mundil, R., Campbell, I.H., 

Korsuch, R.J., Williams, I.S., and Foudoulis, C, 2004, 

Improved 206Pb/238U microprobe geochronology by the 

monitoring of a trace-element-related matrix effect; 

SHRIMP, ID–TIMS, ELA–ICP–MS and oxygen isotope 

documentation for a series of zircon standards: Chemical 

Geology, v. 205, p. 115–140.

Brimhall, G.H., Jr., 1973, Mineralogy, texture, and chemistry 

of early wall rock alteration in the deep underground mines 

and Continental area, Butte district, Montana, in Miller, 

R.N., ed., Guidebook for the Butte field meeting of Society 

of Economic Geologists: Littleton, Colo., Society of Economic Geologists, p. H1–H5.

Brimhall, G.H., Jr., 1977, Early fracture-controlled disseminated mineralization at Butte, Montana: Economic Geology, 

v. 72, p. 37–59.
36 Synthesis of Petrographic, Geochemical, and Isotopic Data for the Boulder Batholith, Southwest Montana

Brimhall, G.H., Jr., 1979, Lithologic determination of mass 

transfer mechanisms of multiple-stage porphyry copper 

mineralization at Butte Montana—Vein formation by hypogene leaching and enrichment of potassium-silicate protore: 

Economic Geology, v. 74, p. 556–589.

Brimhall, G.H., Jr., 1980, Deep hypogene oxidation of porphyry copper potassium-silicate protore at Butte, Montana—A theoretical evaluation of the copper remobilization 

hypothesis: Economic Geology, v. 75, p. 384–409.

Brown, R.G., 1894, The ore deposits of Butte City: Transactions of the American Institute of Mining and Metallurgical 

Engineering, v. 24, p. 543–558.

Cameron, K.L., and Cameron, M., 1985, Rare earth element, 87Sr/86Sr, and 143Nd/144Nd compositions of Cenozoic orogenic dacites from Baja California, northwestern Mexico, 

and adjacent West Texas; evidence for the predominance of 

a subcrustal component: Contributions to Mineralogy and 

Petrology, v. 91, p. 1–11.

Christiansen, R.L., and Yeats, R.S., 1992, Post-Laramide 

geology of the U.S. Cordillera region, in Burchfiel, B.C., 

Lipman, P.W., and Zoback, M.L., eds., The Cordilleran 

Orogen—Conterminous U.S.: Boulder, Colo., Geological 

Society of America, The Geology of North America, v. G–3, 

p. 261–406.

Coney, P.J., and Reynolds, S.J., 1977, Cordilleran Benioff 

zones: Nature, v. 270, p. 403–406.

Davis, W.E., Kinoshita, W.T., and Smedes, H.W., 1963, Bouguer gravity, aeromagnetic, and generalized geologic map 

of East Helena and Canyon Ferry quadrangles and part of 

the Diamond City quadrangle, Lewis and Clark, Broadwater, and Jefferson counties, Montana: U.S. Geological 

Survey Geophysical Investigations Map GP–444, scale 

1:62,500.

Davis, W.E., Kinoshita, W.T., and Robinson, G.D., 1965, 

Bouguer gravity, aeromagnetic, and generalized geologic 

map of the western part of the Three Forks Basin, Jefferson, Broadwater, Madison, and Gallatin counties, Montana: 

U.S. Geological Survey Geophysical Investigations Map 

GP–497, scale 1:62,500.

Davidson, Jon, Turner, Simon, Handley, Heather, Macpherson, 

Colin, and Dosseto, Anthony, 2007, Amphibole “sponge” in 

arc crust?: Geology, v 35, p. 787–790.

Doe, B.R., and Tilling, R.I., 1967, The distribution of lead 

between coexisting K-feldspar and plagioclase: American 

Mineralogist, v. 52, p. 805–816.

Doe, B.R., Tilling, R.I., Hedge, C.E., and Klepper, M.R., 

1968, Lead and strontium isotope studies of the Boulder 

batholith, southwestern Montana: Economic Geology, v. 63, 

no. 8, p. 884–906.

du Bray, E.A., John, D.A., Putirka, Keith, and Cousens, B.L., 

2009, Geochemical database for igneous rocks of the ancestral Cascades arc—Southern segment, California and Nevada: 

U.S. Geological Survey Data Series 439, 1 CD-ROM.

du Bray, E.A., John, D.A., Sherrod, D.R., Evarts, R.C., Conrey, R.M., and Lexa, Jaroslav, 2006, Geochemical database 

for volcanic rocks of the Western Cascades, Washington, 

Oregon, and California: U.S. Geological Survey Data Series 

155, 49 p.

du Bray, E.A., Ludington, S., Brooks, W.E., Gamble, B.M., 

Ratte, J.C., Richter, D.R., and Soria-Escalante, E., 1995, 

Compositional characteristics of middle to upper Tertiary 

volcanic rocks of the Bolivian Altiplano: U.S. Geological 

Survey Bulletin 2119, 30 p.

du Bray, E.A., Lund, Karen, Tilling, R.I., Denning, P.D., and 

DeWitt, Ed, 2009, Geochemical database for the Boulder 

batholith and its satellitic plutons, southwest Montana: U.S. 

Geological Survey Data Series 454, 1 CD-ROM.

Elliott, J.E., Wallace, C.A., Lee, G.K., Antweiler, J.C., Lidke, 

D.J., 1992, Maps showing mineral resource assessment for 

vein and replacement deposits of gold, silver, copper, lead, 

zinc, manganese and tungsten in the Butte 1 x 2 degree 

quadrangle, Montana: U.S. Geological Survey I–2050–D, 

scale 1:250,000.

Emmons, S.F., and Tower, G.W., Jr., 1897, Economic geology of the Butte special district [Montana]: U.S. Geological 

Survey Atlas, Butte Folio no. 38, p. 3–8.

Emmons, S.F., and Weed, W.H., 1897, Butte Folio: U.S. Geological Survey Atlas, no. 38, 8 p.

Ewart, Anthony, 1982, The mineralogy and petrology of 

Tertiary-Recent orogenic volcanic rocks with special 

reference to the andesitic-basaltic compositional range, in 

Thorpe, R.S., ed., Andesites: Chichester, Wiley, p. 25–87.

Feeley, T.C., and Davidson, J.P., 1994, Petrology of calcalkaline lavas at Volcan Ollague and the origin of compositional diversity at central Andean stratovolcanoes: Journal 

of Petrology, v. 35, p. 1295–1340.

Fleck, R.J., 1990, Neodymium, strontium, and trace-element 

evidence of crustal anatexis and magma mixing in the Idaho 

batholith, in Anderson, J.L. ed., The nature and origin of 

Cordilleran magmatism: Boulder, Colorado, Geological 

Society of America Memoir 174, p. 359–373.

Fleck, R.J., and Criss, R.E., 1985, Strontium and oxygen isotopic variations in Mesozoic and Tertiary plutons of central 

Idaho: Contributions to Mineralogy and Petrology, v. 90, 

p. 291–308.

Fleck, R.J., and Criss, R.E., 2004, Location, age, and tectonic 

significance of the western Idaho suture zone (WISZ): U.S. 

Geological Survey Open-File Report 2004–1039, 48 p.
References Cited 37

Foster, D.A., Mueller, P.A., Mogk, D.W., Wooden, J.L., and 

Vogl, J.J., 2006, Proterozoic evolution of the western margin of the Wyoming Craton—implications for the tectonic 

and magmatic evolution of the Northern Rocky Mountains: 

Canadian Journal of Earth Sciences, v. 43, p. 1601–1619.

Frost, B.R., Barnes, C.G., Collins, W.J., Arculus, R.J., Ellis, 

D.J., and Frost, C.D., 2001, A geochemical classification for 

granitic rocks: Journal of Petrology, v. 42, p. 2033–2048.

GEOROC, 2007, Geochemistry of rocks of the oceans and 

continents: Max Plank Institute für Chemie, accessed May 

2011 at http://georoc.mpch-mainz.gwdg.de/georoc/.

Gill, James, 1981, Orogenic andesites and plate tectonics: 

New York, Springer-Verlag, 390 p.

Gottfried, David, Rowe, J.J., and Tilling, R.I., 1972, Distribution of gold in igneous rocks: U.S. Geological Survey 

Professional Paper 727, 42 p.

Greenland, L.P., Gottfried, David, and Tilling, R.I., 1968, 

Distribution of manganese between coexisting biotite and 

hornblende in plutonic rocks: Geochimica et Cosmochimica 

Acta, v. 32, p. 1149–1163.

Greenland, L.P., Gottfried, David, and Tilling, R.I., 1974, Iridium in some calcic and calc-alkalic batholithic rocks of the 

western United States: Chemical Geology, v. 14, p. 117–122.

Greenland, L.P., Tilling, R.I., and Gottfried, David, 1971, 

Distribution of cobalt between coexisting biotite and hornblende in igneous rocks: Neues Jahrbuch für Mineralogie, 

Monatshefte, v. 1, p. 33–42.

Gunn, S.H., 1991, Isotopic constraints on the crustal evolution 

of southwest Montana: Santa Cruz, Calif., University of 

California at Santa Cruz Ph.D. dissertation, 190 p.

Hamilton, Warren, and Myers, W.B., 1967, The nature of 

batholiths: U.S. Geological Survey Professional Paper 

554–C, 30 p.

Hanson, G.N., 1980, Rare earth elements in petrogenetic 

studies of igneous systems: Annual Review of Earth and 

Planetary Sciences, v. 8, p. 371–406.

Hildreth, Wes, and Moorbath, Stephen, 1988, Crustal contributions to arc magmatism in the Andes of central Chile: Contributions to Mineralogy and Petrology, v. 98, p. 455–489.

Irvine, T.N., and Baragar, W.R.A., 1971, A guide to the chemical classification of the common volcanic rocks: Canadian 

Journal of Earth Sciences, v. 8, p. 523–548.

John, D.A., Ayuso, R.A., Barton, M.D., Blakely, R.J., Bodnar, 

R.J., Dilles, J.H., Gray, Floyd, Graybeal, F.T., Mars, J.C., 

McPhee, D.K., Seal, R.R., Taylor, R.D., and Vikre, P.G., 

2010, Porphyry copper deposit model, chap. B of Mineral 

deposit models for resource assessment: U.S. Geological 

Survey Scientific Investigations Report 2010–5070–B, 169 p.

Klepper, M.R., Robinson, G.D., and Smedes, H.W., 1971a, On 

the nature of the Boulder batholith of Montana: Geological 

Society of America Bulletin, v. 82, p. 1563–1580.

Klepper, M.R., Robinson, G.D., and Smedes, H.W., 1974, 

Nature of the Boulder batholith of Montana—Discussion: 

Geological Society of America Bulletin, v. 85, p. 1953–1960.

Klepper, M.R., Ruppel, E.T., Freeman, V.L., and Weeks, 

R.A., 1971b, Geology and mineral deposits, east flank of 

the Elkhorn Mountains, Broadwater county, Montana: U.S. 

Geological Survey Professional Paper 665, 66 p.

Klepper, M.R., Weeks, R.A., and Ruppel, E.T., 1957, Geology 

of the southern Elkhorn Mountains, Jefferson and Broadwater counties, Montana: U.S. Geological Survey Professional 

Paper 292, 82 p.

Knopf, Adolph, 1913, Ore deposits of the Helena mining region, 

Montana: U.S. Geological Survey Bulletin 527, 143 p.

Knopf, Adolph, 1950, The Marysville Granodiorite stock, 

Montana: American Mineralogist, v. 35, p. 834–845.

Knopf, Adolph, 1956, Argon-potassium determination of the 

age of the Boulder bathylith, Montana: American Journal of 

Science, v. 254, p. 744–745.

Knopf, A., 1957, The Boulder bathylith of Montana: American 

Journal of Science, v. 255, p. 81–103.

Knopf, Adolph, 1963, Geology of the northern part of the 

Boulder bathylith and adjacent area, Montana: U.S. Geological Survey Miscellaneous Geologic Investigations I–381, 

scale 1:48,000.

Knopf, Adolph, 1964, Time required to emplace the Boulder 

bathylith, Montana, a first approximation: American Journal 

of Science, v. 262, p. 1207–1211.

Lambe, R.N., 1981, Crystallization and petrogenesis of 

the southern portion of the Boulder Batholith, Montana: 

Berkeley, Calif., University of California Ph.D. dissertation, 171 p.

Lange, I.M., and Cheney, E.S., 1971, Sulfur isotopic reconnaissance of Butte, Montana: Economic Geology, v. 66, 

p. 63–74.

Le Maitre, R.W., 1989, A classification of igneous rocks and 

glossary of terms: Oxford, Blackwell Scientific, 193 p.

Lipman, P.W., 1992, Magmatism in the Cordilleran United 

States; Progress and problems, in Burchfiel, B.C., Lipman, 

P.W., and Zoback, M.L., eds., The Cordilleran orogen—

Conterminous U.S.: Geological Society of America, The 

Geology of North America, v. G–3, p. 481–514.

Ludwig, K.R., 2001, Squid, version 1.02, A user’s manual: 

Berkeley Geochronology Center Special Publication, 

No. 2, 21 p.
38 Synthesis of Petrographic, Geochemical, and Isotopic Data for the Boulder Batholith, Southwest Montana

Ludwig, K.R., 2003, Isoplot/Ex version 3.00, A geochronological toolkit for Microsoft Excel: Berkeley Geochronology 

Center Special Publication, no. 4, 73 p.

Lund, Karen, editor, 2007, Earth science studies in support of 

public policy development and land stewardship; Headwaters Province, Idaho and Montana: U.S. Geological Survey 

Circular 1305, 92 p.

Lund, Karen, Aleinkoff, J.N., Kunk, M.J., Unruh, D.M., 

Zeihen, G.D., Hodges, W.C., du Bray, E.A., and O’Neill, 

J.M., 2002, Shrimp U-Pb and 40Ar/39Ar age constraints 

for relating plutonism and mineralization in the Boulder 

batholith region, Montana: Economic Geology, v. 97, 

p. 241–267.

Meyer, C., 1965, An early potassic type of wall-rock alteration at Butte, Montana: American Mineralogist, v. 50, 

p. 1717–1722.

Meyer, C., Shea, E.P., Goddard, C.C., Jr., Zeihen, L.G., 

Guilbert, J.M., Miller, R.N., McAleer, J.F., Brox, G.B., 

Ingersoll, R.G., Jr., Burns, G.J., and Wigal, T., 1968, Ore 

deposits at Butte, Montana, in Ridge, J.D., ed., Ore deposits of the United States, 1933–1967 (Graton-Sales v. 2): 

New York, American Institute of Mining and Metallurgical 

Engineers, p. 1373–1416.

Miller, R.N., 1973, Production history of the Butte district 

and geological function, past and present, in Miller, R.N., 

ed., Guidebook for the Butte field meeting of Society of 

Economic Geologists: Littleton, Colo., Society of Economic 

Geologists, p. F1–F10.

Miyashiro, Akiho, 1974, Volcanic rock series in island arcs 

and active continental margins: American Journal of Science, v. 274, p. 321–355.

O’Neill, J.M., Lund, Karen, Van Gosen, B.S., Desborough, 

G.A., Sole, T.C., and DeWitt, E.H., 2004, Geologic framework, in Nimick, D.A., Church, S.E., and Finger, S.E., eds, 

Integrated investigations of environmental effects of historical mining in the Basin and Boulder Mining Districts, 

Boulder River watershed, Jefferson county, Montana: U.S. 

Geological Survey Professional Paper 1652, p. 49–88.

Pearce, J.A., Harris, N.B.W., and Tindle, A.G., 1984, Trace 

element discrimination diagrams for the tectonic interpretation of granitic rocks: Journal of Petrology, v. 25, 

p. 956–983.

Proffett, J.M., 1973, Structure of the Butte district, Montana, 

in Miller, R.N., ed., Guidebook for the Butte field meeting 

of Society of Economic Geologists: Littleton, Colo., Society 

of Economic Geologists, p. G1–G12.

Richards, J.P., and Kerrich, R., 2007, Adakite-like rocks: their 

diverse origins and questionable role in metallogenesis: 

Economic Geology, v. 102, p.537–576.

Roberts, S.A., 1975, Early hydrothermal alteration and mineralization in the Butte district, Montana: Cambridge, Mass., 

Harvard University Ph.D. dissertation, 157 p.

Robinson, G.D., Klepper, M.R., and Obradovich, J.D., 1968, 

Overlapping plutonism, volcanism, and tectonism in the 

Boulder batholith region, western Montana in Coats, 

R.R., Hay, R.L., and Anderson, C.A., eds., Studies in 

volcanology: Geological Society of America Memoir 

116, p. 557–576.

Ruppel, E.T., 1963, Geology of the Basin quadrangle, Jefferson, Lewis and Clark, and Powell counties, Montana: U.S. 

Geological Survey Bulletin 1151, 121 p.

Rusk, B.G., Reed, M.H., and Dilles, J.H., 2008, Fluid inclusion evidence for magmatic-hydrothermal fluid evolution in 

the porphyry copper-molybdenum deposit at Butte, Montana: Economic Geology, v. 103, p. 307–334.

Rutland, C., Smedes, H.W., Tilling, R.I., and Greenwood, 

W.R., 1989, Volcanism and plutonism at shallow crustal 

levels—The Elkhorn Mountains Volcanics and the Boulder 

batholith, southwestern Montana: International Geological 

Congress, 28th, Washington, D.C., Field Trip Guidebook 

T337, p. 16–31.

Sales, R.H., 1913, Ore deposits at Butte, Montana: Transactions of the Society of Mining Engineers of American 

Institute of Mining, Metallurgical and Petroleum Engineers, 

Incorporated, p. 1523–1626.

Sales, R.H., and Meyer, C., 1948, Wall-rock alteration, Butte, 

Montana: Transactions of the American Institute of Mining, 

Metallurgical and Petroleum Engineers, v. 178, p. 9–33.

Sales, R.H., and Meyer, C., 1949, Results for preliminary studies of vein formation at Butte, Montana: Economic Geology, v. 44, p. 465–484.

Sawka, W.N., Chappell, B.W., and Norrish, Keith, 1984, 

Light-rare-earth-element zoning in sphene and allanite during granitoid fractionation: Geology, v. 12, p. 131–134.

Schmidt, C.J., Smedes, H.W., and O’Neill, J. M., 1990, Syncompressional emplacement of the Boulder and Tobacco 

Root batholiths (Montana—USA) by pull-apart along old 

fault zones: Geological Journal, v. 25, p. 305–318.

Shand, S.J., 1951, Eruptive rocks: New York, John Wiley, 488 p.

Smedes, H.W., 1966, Geology and igneous petrology of the 

northern Elkhorn Mountains, Jefferson and Broadwater 

counties, Montana: U.S. Geological Survey Professional 

Paper 510, 116 p.

Smedes, H.W., Klepper, M.R., and Tilling, R.I., 1973, The 

Boulder batholith, Montana, in Miller, R.N., ed., Guidebook for the Butte field meeting of Society of Economic 

Geologists, 1973: Butte, Montana, Society of Economic 

Geologists, p. E1–20.
References Cited 39

Smedes, H.W., Klepper, M.R., and Tilling, R.I., 1988, Preliminary map of plutonic units of the Boulder batholith, 

southwestern Montana: U.S. Geological Survey Open-File 

Report 88–283, scale 1:200,000.

Stacey, J.S., and Kramers, J.D., 1975, Approximation of terrestrial lead isotope evolution by a two-stage model: Earth 

and Planetary Science Letters, v. 26, p. 207–226.

Steiger, R.H., and Jäger, E., 1977, Subcommission on geochronology; Convention on the use of decay constants in 

geo- and cosmochronology: Earth and Planetary Science 

Letters, v. 36, p. 359–362.

Streckeisen, Albert, 1976, To each plutonic rock its proper 

name: Earth-Science Reviews, v. 12, p. 1–33.

Sun, S.-S., and McDonough, W.F., 1989, Chemical and 

isotopic systematics of oceanic basalts; implications for 

mantle composition and processes in Saunders, A.D., 

and Norry, M.J., eds., Magmatism in the ocean basins: 

Geological Society of London Special Publications, v. 42, 

p. 313–345.

Taylor, C.D., Winick, J.A., Unruh, D.M., and Kunk, M.J., 

2007, Geochronology and geochemistry of the IdahoMontana porphyry belt in Lund, Karen, ed., Earth science 

studies in support of public policy development and land 

stewardship—Headwaters Province, Idaho and Montana: 

U.S. Geological Survey Circular 1305, p. 26–39.

Tilling, R.I., 1964, Variation in modes and norms of an 

“Homogeneous” pluton of the Boulder batholith, Montana: U.S. Geological Survey Professional Paper 501–D, 

p. D8–D13.

Tilling, R.I., 1968, Zonal distribution of variations in structural state of alkali feldspars within the Rader Creek pluton, 

Boulder batholith, Montana: Journal of Petrology, v. 9, no. 

3, p. 331–357.

Tilling, R.I., 1973, Boulder batholith, Montana—A product 

of two contemporaneous but chemically distinct magma 

series: Geological Society of America Bulletin, v. 84, 

p. 3879–3900.

Tilling, R.I., 1974, Composition and time relations of plutonic 

and associated volcanic rocks, Boulder batholith region, 

Montana: Geological Society of America Bulletin, v. 85, 

p. 1925–1930.

Tilling, R.I., 1977, Interaction of meteoric waters with magmas of the Boulder batholith, Montana: Economic Geology, 

v. 72, no. 5, p. 859–864.

Tilling, R.I., and Gottfried, David, 1969, Distribution of 

thorium, uranium, and potassium in igneous rocks of the 

Boulder batholith region, Montana, and its bearing on 

radiogenic heat production and heat flow: U.S. Geological 

Survey Professional Paper 614–E, p. E1–E29.

Publishing support provided by: 

Denver Publishing Service Center

For more information concerning this publication, contact:

Center Director, USGS Central Mineral and Environmental Resources 

Science Center

Box 25046, Mail Stop 973

Denver, CO 80225

(303) 236-1562

Or visit the Central Mineral and Environmental Resources Science 

Center Web site at:

http://minerals.cr.usgs.gov/

Tilling, R.I., Klepper, M.R., and Obradovich, J.D., 1968, 

K-Ar ages and time span of emplacement of the Boulder 

batholith, Montana: American Journal of Science, v. 266, 

p. 671–689.

Turekian, K.K., and Wedepohl, K.H., 1961, Distribution of the 

elements in some major units of the Earth’s crust: Geological Society of America Bulletin, v. 72, p. 175–192.

Tysdal, R.G., 2000, Revision of Cretaceous Slim Sam Formation, Elkhorn Mountains area, Montana: U.S. Geological 

Survey Professional Paper 1601–B, 8 p.

Tysdal, R.G., Ludington, Steve, and McCafferty, A.E., 1996, 

Mineral and energy resource assessment of the Helena 

National Forest, west-central Montana: U.S. Geological 

Survey Open-File Report 96–683–A, 322 p., 6 plates.

Unruh, D.M., Lund, Karen, Snee, L.W., and Kuntz, M.A., 

2008, Uranium-lead zircon ages and Sr, Nd, and Pb isotope 

geochemistry of selected plutonic rocks from western 

Idaho: U.S. Geological Survey Open-File Report 2008–

1142, 36 p.

Wark, D.A., 1991, Oligocene ash flow volcanism, northern 

Sierra Madre Occidental; role of mafic and intermediatecomposition magmas in rhyolite genesis: Journal of Geophysical Research, v. 96, p. 13,389–13,411.

Weed, W.H., 1912, Geology and ore depositions of the Butte 

district, Montana: U.S. Geological Survey Professional 

Paper 74, 262 p.

Wood, D.A., Joron, J.L., and Treuil, Michel, 1979, A reappraisal of the use of trace elements to classify and 

discriminate between magma series erupted in different 

tectonic settings: Earth and Planetary Science Letters, v. 45, 

p. 326–336.
du Bray and others—Synthesis of Petrographic, Geochemical, and Isotopic Data for the Boulder Batholith, Southwest Montana—Professional Paper 1793